Learned about LESS yesterday, even for people who aren't CSS nerds (I'm not) this makes editing CSS a lot more powerful, and is definitely worth a look for every web developer. There is also a handy OS X app to accompany it.

HipHop for PHP isn't technically a compiler itself. Rather it is a source code transformer. HipHop programmatically transforms your PHP source code into highly optimized C++ and then uses g++ to compile it.

While not the first attempt to compile PHP code, this comes from one of the largest companies utilizing PHP on the web, so it's one to keep an eye on.