The leading online brokerages for exchange traded fund investors offer free ETF trades or dirt-cheap rates, according to IBD’s survey of investors. This lets successful investors dollar-cost average or rebalance frequently at no cost just as they do with mutual funds .

ETF providers are covering the cost of trades to attract investors to their funds rather than to competitors, which in many cases track the same index.

Investors should beware the fine print. Some brokers charge fees for frequent trading, have account minimums or offer free trades on a short list of ETFs.

Here’s a look at five online brokers most favored by respondents to IBD’s Best Online Brokers survey and their ETF trading perks.

• Thinkorswim. This platform from TD Ameritrade (NYSE:AMTD ), blows away competitors in the ETF arena, offering the widest array of commission-free trades on 37 funds from iShares, 32 from Vanguard, 12 from the SPDR family, and one each from Barclays, Deutsche Bank, Pimco, PowerShares, Market Vectors and WisdomTree.

The firm charges a $19.99 fee for short-term ETF trades if you sell a position within 30 days of buying it. It’s the only firm on this list that has no minimum for regular accounts. The minimum for margin accounts is $2,000. All other trades cost $9.99.

• Interactive Brokers. This company (NASDAQ:IBKR ) provides free trading on 31 Global X Funds and five FactorShares. But it requires the highest account minimum $10,000 of any other brokers in this list.

The minimum for IRA accounts is $5,000. It charges a $10 minimum monthly activity fee, so no matter what, you will be charged at least $10 a month in commissions.

Otherwise, it costs half a cent per share per trade, or a minimum of $1, with a maximum of 0.5% of the trade.

• Fidelity Investments. This giant gives free trades on 30 widely traded iShares ETFs and its one ETF, Fidelity Nasdaq Composite Index Tracking Stock Fund (NASDAQ:ONEQ ).

Otherwise, commissions run $7.95 a trade. The minimum account requirement is $2,500.

Fidelity says iShares ETFs are not margin eligible for 30 days from settlement, which means those shares can count toward your margin-buying power after 30 days.

• Charles Schwab. This firm (NYSE:SCHW ) lets investors trade 15 Schwab ETFs as often as they want, with no penalties for short-term trades. Normal commission fee is $8.95. The account minimum is $1,000.

• TradeStation. The firm offers no free ETF trading, but charges merely 1 cent per share for up to 500 shares and 0.6 cent a share thereafter for all trades. Regular commissions run $6.99 to $9.99 depending on the order size and type. It requires a $5,000 minimum for regular accounts and a $30,000 minimum for day-trading accounts.