Ever since reading "The Little Schemer", I knew I had to write my own implementation of scheme. I started, and I didn't get very far before I noticed someone else had the same idea, and a better implemented one at that.
"bus-scheme" is the name, and I'm loving working with it [on the MAX]. Definitely worth checking out if you're interested in compilers, languages, scheme, or any combination thereof.
(In case you're curious, here's the version that I started.)