Beast: An open source Rails forum in under 500 lines of code: The link text really says it all.
Rick Olson and Josh Goebel have teamed up to produce Beast, a lightweight but well styled forum. Rails has sorely been lacking a good, well styled, open source forum, but Beast is great.
Ruby is as concise as all get-out. Aided by The Pickaxe Book, I once wrote a Web server in about 20 lines of code. It was just a little script that listened on a specified port and would open files and return the contents. I had no need for it, but it was fun to play with and really demonstrated how powerful and fast Ruby could be.