If you want to just have a play around with some algos check out...

  1. 25 Posts.
    If you want to just have a  play around with some algos check out https://www.quantopian.com/

    They have contests, their own api and also free backtesting.

    You can use this to test your own trading strats while/if you are making your own platform from scratch.

    Cons are you obviously have to use their api but great way to get started

    (Note I am not affiliated with them, just passing on something i found fun and useful)
 
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