Lean Algorithmic Trading Engine by QuantConnect (C#, Python, F#, VB, Java)
Python for .NET is a package that gives Python programmers nearly seamless integration with the .NET Common Language Runtime (CLR) and provides a powerful application scripting tool for .NET developers.
Automation of Interactive Brokers TWS. You can download the latest release here: https://github.com/ib-controller/ib-controller/releases/latest
Visual Studio plugin utilizing the QuantConnect REST API to launch backtests and display results inside Visual Studio 2010-2014.
Cross platform implementation of FXCM Java Trading API in .NET for connecting with FXCM. Uses IKVM to port the Java library to a .NET DLL.
Pusher .NET client library for interacting with the Pusher WebSocket API