And don't forget distcc [1], together they can speed up compilations and recompilations. I remember using colleague computers to speed up a 30 min C++ compilation to just 5 min... to just seconds on second compilation.
When Gentoo was hip new thing we put a bunch of our office machines into distcc cluster just for that.
Distcc is great when it works but just yesterday I had to make the admin reboot my VM three times before giving up: my VM used all its memory which made it unusable..