Almost forgot: There's also a reading guide for the Lua source (http://www.reddit.com/comments/63hth/ask_reddit_which_oss_co...) by Mike Pall, the author of LuaJIT (http://luajit.org/). The source for that is almost certainly a goldmine too (it's a cutting-edge JIT compiler, after all), but I haven't really looked at its internals yet.
(Also, should be "simple-but-efficient garbage collection" above.)
IoL4 (http://www.iol4.com/) is an L4-based microkernel that boots into Io (http://www.iolanguage.com/), which is another language that looks rather interesting. I've never got the Io VM to build on OpenBSD/amd64, which rules it out for me, though.
I'd like to do that with a Forth or a Lisp one of these days, time permitting.
(Also, should be "simple-but-efficient garbage collection" above.)