BubbleWrap is “a collection of (tested) helpers and wrappers used to wrap CocoaTouch code and provide more Ruby like APIs” for RubyMotion. There is a ton of great stuff in BubbleWrap.
One of the most interesting things I’ve seen in awhile is EventMachine implemented using GCD.
If you’re doing any RubyMotion work, I think you should definitely checkout BubbleWrap!