Xubuntu is a variant of Ubuntu that uses the lightweight Xfce desktop environment, making it a good choice for low-powered systems.
Or so they say. So I dusted off (literally) a Gateway Solo laptop (Pentium II, 300 MHz, 64 MB RAM) and replaced its Windows 98 with Xubuntu 6.06. And that's what I'm posting from now! Go, Xubuntu! The only problem is that a naughty kitten tore several keys out of the keyboard several years ago, making typing difficult. And he would have to get the 'e', the little stinker.
I'm using a 2-year-old wireless card with it, though. I failed with my first attempt to make its old ethernet card work, and decided to take Tim Almond's advice and just use a known compatible device rather than go into ethernet-card archaeology.
Hmm, now there's a new working laptop in the house. Oh, the possibilities...