• Rabbit MQ – Rabbit MQ is a scalable message/queue AMQP server with WCF/C# client (free)
  • Twillio – This is an awesome commercial programmable VOIP infrastructure. Free accounts available (commercial)
  • ejabberd – This is one really scalable jabber/XMPP server (free)
  • fyiReporting – This is a free reporting tools for .Net based on Report Definition Langauge (RDL) (free)
  • Dotnet OpenID – C# library for OpenID authentication (free)
  • Tweetsharp – An excellent library for Tweeter API (free)
  • XML-RPC.Net – This is *the* library to make XML-RPC calls (free)
  • Jabber-net – This is the only client library for Jabber/XMPP protocol
  • Mono RelaxNG Validator – This is the most viable RelaxNG validator reader for .Net (free)
  • Quartz. Net – Enterprise job scheduler for .Net (free)
  • Topshelf – Windows service application framework (free)
  • Facebook Developer Toolkit – If you want to develop a Facebook App on .Net, use this (free)
  • Gitsharp – Library for Git version control (free)
  • SharpSVN – A library for Subversion Client API (free)
  • Math.NET – A mathematical open source (MIT/X11, LGPL & GPL) library written in C#/.Net, aiming to provide a self contained clean framework for symbolic algebraic and numerical / scientific computations. (free)
  • Linq to Twitter – Linq to Twitter
  • Sharp SSH – SSH Implementation in C#