Interactive Brokers also offer a paper trading account that can be traded via their API just like a regular account, although you need to have an account with them.

There are some limitations on specific order type/venue combinations, but most things are supported.

https://www.interactivebrokers.com/en/index.php?f=tws&p=pape...

I've looked at them before, but I don't have $10k to tie up on what is just me playing around.

Nah bro. You can try their demo account. The login is 'edemo/demouser' and you can use it via their API as well. The only issue is that the demo account is not persistent, so you'll lose your positions the next trading day lol. Plus there are tons of peeps sharing the account trying to test their trading bot as well after-hours. But if you're just doing daytrading QA, it's pretty good.

http://www.quantopian.com let's you paper trade using algorithms if that's what you're after

quantopian looks nice. I'm glad someone is in this niche.

There is no substitute for the real thing. Get an account, build the algo, do the best you can to minimize maximum damage, and run with it.