NET

Gardens Point Ruby.NET Compiler
Compiler targeting Microsoft .NET platform; can statically compile source file into verifiable .NET v2.0 assembly or directly execute source file: compile, load and execute.
http://plas.fit.qut.edu.au/ruby.net/

IronRuby
A .NET Ruby, goals: good performance, seamless integration with .NET libraries, infrastructure; compatibility with 1.8.x branch of Ruby modulo continuations. Heavily leverages Microsoft Dynamic Language Runtime. Microsoft Permissive License.
http://www.ironruby.net/

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/

RubyForge: RubyCLR: Project Info
Official home; downloads, latest versions.
http://rubyforge.org/projects/rubyclr/

Ruby.NET
Project to make Ruby compiler for .NET based on NetRuby.
http://jaen.saul.ee/index.php/2005/07/01/rubynet-proposal/

Third Drop of RubyCLR
High-performance Ruby to .NET bridge, allows seamless integration of CLR and Ruby objects in the same Win32 process, allows rich client programs using Windows Forms or Presentation Foundation libraries. Author web site/blog.
http://www.iunknown.com/2006/03/third-drop-of-r.html

IronRuby vs. Ruby.NET?
Response to M. David Peterson posting. John Lam on Software. <small>(January 2, 2008)</small>
http://www.iunknown.com/2008/01/ironruby-vs-rub.html

Ruby.NET vs. IronRuby: What's the Difference?
Brief updated comparison, by M. David Peterson. O'Reilly Windows DevCenter Blog. <small>(January 1, 2008)</small>
http://www.oreillynet.com/windows/blog/2008/01/rubynet_vs_ironruby_whats_the.html

A First Look at IronRuby
Basic description, links. John Lam on Software. <small>(July 23, 2007)</small>
http://www.iunknown.com/2007/07/a-first-look-at.html