[aaronbrethorst]

MacRuby is an implementation of Ruby 1.9 built on top of Cocoa. It's also OS X-only. See http://macruby.org

RubyMotion is fully compiled, and built on top of LLVM. See http://www.rubymotion.com/features/

[chc]

They are both compiled and built on top of LLVM. RubyMotion is a fork of MacRuby, not a different design.

[cnp]

And as of last week, with the 2.0 release, you can now build desktop apps using RubyMotion as well. I kind of think that was the nail in the MacRuby coffin, for better or for worse.