The number of exchange traded funds in the stock market pushed past 1,500 this year for the first time as the industry celebrated its 20th anniversary and topped $1 trillion in assets.

The fierce competition for assets has driven ETF providers to partner with brokerages to offer commission-free trading in hopes of attracting inflow and boosting revenue from annual management fees.

Traders check a stock market with ETFs numbering over 1,500. AP View Enlarged Image

But investors must be aware that many ETFs available for free trading are thinly traded and have wide bid-ask spreads, which can add to trading costs.

Also, some brokers charge fees for short-term trades.

Here’s a look at the five rated highest by customers for ETF offerings in IBD’s 2013 Best Online Brokers survey:

• Charles Schwab (NYSE:SCHW ) offers commission-free trading on more than 120 ETFs on its Schwab ETF OneSource platform. Choices include 45 funds from Guggenheim, 31 SPDRs, 21 funds from Schwab, 14 from PowerShares, seven from CurrencyShares, six physical precious metal funds from ETF Securities and four funds from United States Commodity Funds. There are no holding period minimums or monthly service fees.

A $1,000 account minimum is waived if you automatically deposit $100 every month or open a checking account.

An added bonus is that Schwab ETFs carry the lowest expense ratios in the industry.

• Interactive Brokers offers commission-free trading on 29 Global X ETFs. There are no early redemption fees.

If you don’t rack up more than $30 a month in trading commissions, the firm charges a minimum monthly fee of $10 to $30 depending on the account size.

Investors should be aware that Global X ETFs charge anywhere from 45 to 77 basis points of assets in annual management fees.

And fund expenses for their master limited partnership ETFs take total fees to a whopping 3.52% exorbitant compared with the industry average of 0.61%, according to Morningstar.

• Vanguard Brokerage lets you trade all 67 of its ETFs free. But if you buy and sell the same ETF more than 25 times in a 12-month period, you may be barred from trading that ETF for 60 days. Accounts with less than $50,000 are charged a $20 annual service fee.

• TD Ameritrade (NYSE:AMTD ) offers free trading on 101 ETFs from iShares (47), Vanguard (32), Market Vectors (3), PowerShares (4) and SPDRs (12) and one each from iPath, Pimco and WisdomTree.

It charges a short-term trading fee of $19.99 for positions held less than 30 days.

There is no account minimum or monthly service fee.

• TradeStation, geared for active traders, doesn’t offer any commission-free ETF trading.

It charges a penny per share to trade 500 shares and 0.6 cent per share thereafter.

Flat-fee trades cost $4.99 to $9.99 per transaction depending on the number of trades executed per month.

The firm also charges a $99.95 monthly service fee for accounts with less than $100,000 unless they make at least 10 round-trip futures trades or 50 round-trip stock trades or trade at least 5,000 shares or 50 options contracts.