Quantopian - Your algorithmic investing platform
Robinhood - Free stock trading service.
QuantConnect - QuantConnect provides a free algorithm backtesting tool and financial data so engineers can design algorithmic trading strategies. We are democratizing algorithm trading technology to empower investors.
Sumday - Investing how you least expect it
quantra - A public API for quantitative finance made with Quantlib
Matador - Free investing + Social feed of trades + 2.5% on your cash