As I said in IRC, DamnSmall, Puppy, straight Debian, a trimmed Ubuntu, Xubuntu or Fluxbuntu. The last two are Ubuntu that has already been trimmed for use on older systems. Fluxbuntu is not an official Canonical project, but it looks pretty good. It is like DamnSmall with some extra graphical smoothness. You probably don't have enough RAM to run Xubuntu. http://en.wikipedia.org/wiki/Xubuntu

Looks like fluxbuntu.org is currently offline for a redesign, but I could copy the .iso for you from my machine. http://distrowatch.com/table.php?distribution=fluxbuntu http://en.wikipedia.org/wiki/Fluxbuntu

Also lightweight is nUbuntu http://en.wikipedia.org/wiki/NUbuntu for network penetration testing. Also uses Fluxbox.