Ruby.NET - Project to make Ruby compiler for .NET based on NetRuby. - http://jaen.saul.ee/index.php/2005/07/01/rubynet-proposal/
MonoRail - MVC Web framework inspired by RoR Action Pack; fast, readable, simple Ruby-based, almost as powerful as Ruby but for a static (compiled) programs. Part of Castle Project. - http://www.castleproject.org/monorail/
Ruby/.NET Bridge - Allows using Ruby and .NET objects together in programs, access Ruby objects from .NET and vice-versa. Description, tutorial, download, history. - http://www.saltypickle.com/rubydotnet/