When size really matters
For even more resource-constrained environments there's Tiny Core Linux (http://tinycorelinux.net). A basic FLWM LiveCD image weighs in at 15MB and it'll run happily in 64MB RAM. Obviously that doesn't give you a lot of functionality, but it has a nice fine-grained package system that you can tailor to get exactly what you want and nothing else.
I'm not sure I'd be game to use it for my primary work machine (mostly because security updates are ad-hoc, AFAICT), but for special-purpose boxen it's hard to get more lightweight that this.