Writing A Compiler in Ruby
Back in March 2008, Vidar Hokstad - a London based Norwegian developer - began to write a series of blog posts on writing a compiler in Ruby from the ground up. Early on, I took objection to some elements of his approach, but it still stands as a great series of posts. Vidar recently reached post 11, providing enough of a landmark to introduce the series as a whole (which is already scheduled to go up to at least 20 posts).
It's worth noting that there are many different approaches to writing compilers of all types - so don't take the series as a definitive way to develop a compiler. Vidar focuses on outputting x86 assembly language and his example compiler (so far) has very tightly bound AST-walking to code generation stages (with no optimization or intermediate stages).
I'd be particularly intrigued to see similar content from the Ruby community - especially on targeting virtual machines (such as YARV or the Rubinius VM) or on using the Treetop library. For those interested in developing compilers generally, StackOverflow provides a lot of great resources.