I already mentioned Fluxbuntu and Minibuntu. Also Ubuntu can be installed as a minimal system, using alternate CD to install a command-line system.You will find more information about building a custom lightweight system on this page ginving information about installing Ubuntu on low memory systems (Pentium III and earlier machines, with 32-192 MB RAM).
Unfortunately, this information has not been updated for the newest releases.
Cubuntu is one more alternative for those seeking a lightweight alternative for Ubuntu. It is a command-line desktop including email clients, web browsers and a media player.