Guys, You should try quantconnect. I have been using them for a few months. They just anounced live trading too. The platfrom is free
I'm surprised no body recommended Tradestation. Sure it still might be considered a retail trade platform, but it is the most sophisticated and best supported platform of any mentioned here. I have run a hedge fund powered by Tradestation strategies. I see no advantage to developing a platform based on open source, unless you are planning to start your own firm, you will be groaning when you need support, and finding something with good integrated charting will be a chore. The only other suggestion I have is IQBroker, they are working a deal with Tradier for release very soon. Trader has a restful API, but no platform of their own. But that's all new, and I don't know if I would feel comfortable trading my money with such a new platform.
I agree on TradeStation. The downside of open source software is you'll need to spend quite a bit of time tweaking/improving/fixing the code, and that takes time away from strategy design & research. Coding requires a very different mindset than trading and it's very hard to do both. I'm sure some can, but I personally find the two mutually exclusive.