I have a trading system I would like to test for the S&P 500, or another very similar instrument. I'm looking to make approximately 10-15 trades per month. I know if you have a Vanguard account, you can trade their ETFs for no commission, but them limit you to 25 trades in a 12 month period. Are there any other brokerage that let you trade certain instruments without paying a comission?
Take a look at Schwab:
Here's a rundown of the various commission-free options out there. Doesn't give any info on maximum number of trades but that shouldn't be very hard to find on the providers' websites.
I think you've misread the weasel words about Vanguard ETF trades:
If you buy and sell the same Vanguard ETF in a Vanguard Brokerage account more than 25 times in a 12-month period, you may be restricted from purchasing that Vanguard ETF through your Vanguard Brokerage account for 60 days.