The impressive pace of expansion in the ETF industry has been well documented; continuous product development has resulted in the launch of hundreds of products each year, and there are now more than 1,600 names in the ETF lineup. But while the depth of the ETF space has increased, the industry remains quite top-heavy; a relatively small number of products accounts for the lions share of total assets.

Most of the largest ETFs are also among the oldest; their impressive asset bases are perhaps more attributable to their seniority than the appeal of the underlying methodologies. In fact, impressive innovation in the industry has given investors plenty of alternatives to the super tickers that dominate the space [For updates on all new ETFs, sign up for the free ETFdb newsletter ].

The largest ETFs have become so popular for a reason; they generally offer cheap, liquid exposure to core asset classes. But for investors willing to dig a bit deeper, there are some potentially exciting alternatives beyond the big dogs at the top:

1. S&P 500 SPDR (SPY )

Overview: This ETF is one of three offering exposure to the S&P 500, a widely followed benchmark of large cap U.S. equities.

ETF Alternatives: Vanguards VOO offers exposure to the same index at a lower price point (0.05%), but the most intriguing alternative may be the Rydex S&P Equal Weight ETF (RSP ). That fund includes the exact same names as SPY, but gives an equal weighting to each component. The result of that seemingly minor difference has been huge in terms of returns: RSP outperformed SPY by more than 11% from 2006 to 2011 [see Free Member Report: How To Pick The Right ETF Every Time ].

2. Gold SPDR (GLD )

Overview: GLD is one of four ETFs that invests in physical gold. In fact, GLD is one of the largest holders of gold bullion in the world; this ETF currently has about $75 billion in assets.

ETF Alternatives: For cost conscious investors, IAU should be rather appealing. That iShares fund holds the exact same asset (physical gold) but charges 0.25% instead of 0.40%. That material differential virtually guarantees that IAU will deliver better returns over the long run [also read 50 Ways To Invest In Gold ].

3. Vanguard Emerging Markets ETF (VWO )

Overview: This popular fund offers cheap, easy access to hundreds of emerging markets stocks.

ETF Alternatives: There are a number of interesting options in the Emerging Markets ETFdb Category ; a couple of our favorites include the Rydex MSCI Equal Weight Emerging Markets ETF (EWEM ), which maintains a much more balanced portfolio, and the GEMS Composite ETF (AGEM ), which features a much smaller portfolio but avoids the quasi-developed economies of South Korea and Taiwan [see also Emerging & Frontier Markets ETFdb Portfolio ].

4. MSCI EAFE Index Fund (EFA )

Overview: This ETF offers exposure to developed markets outside the U.S. including Europe, Japan, and Australia.

ETF Alternatives: There are 14 ETFs that offer exposure to the EAFE region ; interesting alternatives include the small cap SCZ. equal weighted EWEF. and dividend weighted DWM. Vanguards VEA is linked to the exact same index as EFA, but with a much lower expense ratio.

5. MSCI Emerging Markets Index Fund (EEM )

Overview: This ETF is linked to the same index as VWO .

ETF Alternatives: Investors can save about 52 basis points in fees by switching to VWO. Other alternatives for exposure to emerging markets include the dividend-weighted DEM and the small cap EWX .

6. S&P 500 Index Fund (IVV )

Overview: The second most popular S&P 500 ETF, IVV maintains a slightly different structure than SPY that may be more efficient for buy-and-holders.

ETF Alternatives: Another option for exposure to large cap U.S. stocks is the Equal Sector Weight ETF (EQL ), which maintains equivalent allocations to each of the nine major sectors [see also Equal Weight ETFs: Comparing Similar (But Different) Strategies ].

7. PowerShares QQQ (QQQ )

Overview: This popular trading vehicle offers exposure to the tech-heavy NASDAQ.

ETF Alternatives: First Trusts QQEW is an interesting product; that ETF includes the stocks of the NASDAQ 100, but with an equal weighting (about 1%) assigned to each. The result is a more balanced portfolio that avoids big concentrations in names like Apple and Microsoft.

8. Barclays TIPS Bond Fund (TIP )

Overview: The most popular fund in the Inflation Protected Bonds ETFdb Category. TIP is a low risk fund that has become popular in part because of lingering concerns about inflation.

ETF Alternatives: There is no shortage of ETF options for combating inflation; we like WisdomTrees RRF. which combines exposure to U.S. and international TIPS with commodity exposure [see Inflation ETF Special: 25 Ideas To Fight Rising Prices ].

Short-term TIPS are also intriguing, as these bonds should be less sensitive to interest rate hikes that often accompany climbs in CPI. ETF options in this space include STPZ and STIP .

9. Total Stock Market ETF (VTI )

Overview: For broad exposure to U.S. stock markets, the cost efficient and incredibly deep VTI is tough to beat.

ETF Alternatives: Though the efficiency of VTI is impressive, there are actually options in the All Cap Equities ETFdb Category that are cheaper. FMU from FocusShares and SCHB from Charles Schwab. Other potentially interesting alternatives include a pair of AlphaDEX funds from First Trust: FAB and FAD .

10. iBoxx $ Investment Grade Corporate Bond Fund (LQD )

Overview: This ETF taps into high quality corporate debt, making it useful as a core holding in many portfolios.

ETF Alternatives: There are plenty of choices in the Corporate Bonds ETFdb Category. but two in particular stand out as compelling alternatives to LQD. PFIG from PowerShares and CBND from State Street both implement alternative weighting methodologies that are more intuitively appealing that the cap weighting approach common in fixed income funds [see also Profiling 5 Corporate Bond ETFs Besides LQD ].

11. Barclays Aggregate Bond Fund (AGG )

Overview: This ETF is one of several in the Total Bond Market ETFdb Category that focuses on high quality U.S. debt, including Treasuries and investment grade corporate bonds.

ETF Alternatives: For cheaper exposure to the exact same index, investors actually have three different ETFs from which to choose: BND (0.08%), LAG (0.16%), and SCHZ (at 0.06%, the cheapest bond ETF out there).

12. Russell 2000 Index Fund (IWM )

Overview: This ETF is linked to a broad-based index of small cap U.S. equities.

ETF Alternatives: Vanguards VTWO offers access to the same index at an expense ratio that is 0.01% lower than IWMs, while Rydex offers an equal-weighted version in EWRS .

13. Russell 1000 Growth Index Fund (IWF )

Overview: This ETF includes large cap stocks maintaining growth characteristics, such as higher pricing multiples and expected earnings growth.

ETF Alternatives: The overlap between IWD and its value counterpart is somewhat shocking; dozens of stocks are included in both of these products that might be expected to be mutually exclusive. For purer, exposure to style factors, the Rydex S&P 500 Pure Growth ETF (RPG ) is a better choice here [see all the large cap ETFs here ].

14. Total Bond Market ETF (BND )

Overview: This ETF, which includes investment grade bonds, was already mentioned as a more cost efficient alternative to AGG.

ETF Alternatives: Viewing BND as a one stop shop for fixed income exposure is perhaps a bit short-sighted, as this fund includes no exposure to many of the higher yielding corners of the fixed income market or international securities. The Madrona Global Bond ETF (FWDB ) is much more expensive, but also much more representative of the global fixed income market.

15. Silver Trust (SLV )

Overview: SLV offers exposure to physical silver. which has outperformed gold by a huge margin over the last few years.

ETF Alternatives: For access to physical silver, SIVR is the only other option out there (and is actually quite a bit cheaper than SLV). PowerShares DBS offers exposure to futures-based strategies, while SIL allows investors to access stocks of mining companies engaged in the extraction of the precious metal [see 25 Ways To Invest In Silver ].

16. Barclays 1-3 Year Treasury Bond Fund (SHY )

Overview: This safe haven fund focuses on short-term bonds issued by the U.S. government, an asset class that generally performs well when stocks struggle.

ETF Alternatives: There are dozens of short term bond ETFs ; TUZ offers similar exposure to Treasuries, while CSJ focuses on short-dated investment grade corporate bonds and HYS taps into short term junk bonds.

17. Russell 1000 Value Index Fund (IWD )

Overview: This ETF is the previously mentioned value counterpart to IWF, focusing on large cap stocks with value characteristics such as high dividend yields.

ETF Alternatives: Again, a pure value ETF from Rydex stands out as a potential alternative to IWD; the S&P 500 Pure Value ETF (RPV ) is linked to an index with much stricter value requirements for inclusion [see Under The Hood of Value & Growth ETFs ].

18. Dow Jones Industrial Average ETF (DIA )

Overview: This is the only ETF linked to perhaps the most widely followed stock benchmark in the world: the Dow Jones Industrial Average .

ETF Alternatives: DIA is not the most balanced product in the world; there are only 30 components, and the price-weighted methodology of the underlying benchmark can  result in a top heavy portfolio. There are a handful of other mega cap ETFs. including the iShares S&P 100 Index Fund (OEF ) and Rydex Russell Top 50 ETF (XLG ).

19. MSCI Brazil Index Fund (EWZ )

Overview: The largest country-specific international equity ETF, EWZ offers exposure to many of the largest Brazilian companies.

ETF Alternatives: For those looking to tilt exposure towards smaller companies, both the small cap BRF and mid cap (BRAZ ) ETFs could be interesting. For exposure that is nearly identical to EWZ, the currency hedged DBBR offers access to Brazil without exposure to the value of the Brazilian currency.

20. COMEX Gold Trust (IAU )

Overview: The inclusion of two physical gold ETFs on a list of the 20 largest ETFs highlights how popular this asset class has become.

ETF Alternatives: Besides IAU and GLD. there are two other physical gold ETFs: SGOL holds bullion in Swiss vaults while AGOL stores its gold in Singapore. There are also plenty of options for achieving exposure to gold through stocks of companies that produce and extract the metal; in addition to the ultra-popular GDX. GGGG from Global X offers more of a pure play on gold mining stocks [Can You Do Better Than GDX In The Gold Mining Space? ].

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Disclosure: Long BRF.