Lightweight Linux

Under 50Mb
Distros under 50Mb fit onto a Mini CD.
 * DamnSmallLinux - (<50Mb) popular distro using Knoppix hardware recognition. Based on 2.4.x kernel (puppy uses 2.6.x) Also has good way of running programs using .uci .unc
 * Austrumi - (<50Mb) Austrumi is a business card size bootable live CD Linux distribution based on Slackware.
 * Mitrax - (<50Mb) MitraX is live distro based on Slackware
 * Luit - (<50Mb) with Xfce

Under 128Mb

 * Puppy Linux - (83Mb) see also teenpup, tmxxine, muppy and pizzapup.
 * DamnSmallLinux-Not - (84Mb) popular distro using Knoppix hardware recognition with Abiword and Gnumeric.
 * DeLi - (<128Mb) Desktop Light
 * SLAX Popcorn Edition- (<128Mb) **Quite good**, It's a smaller version of SLAX
 * FeatherLinux - (<128Mb) **not as good**, a Knoppix based distro, can't boot off read-only sticks

Under 256Mb

 * Slax - (~200Mb) Distro based on Slackware using KDE
 * Runt- (~200Mb); future development is uncertain
 * Featherweight- (<250Mb). based on Feather Linux. The goal is to be a full featured Linux that isn't bloated.
 * Flash Linux- (<256Mb) Gentoo-based, initially designed to run off of a 256Mb USB key.
 * AliXe (278Mb) - Slax based
 * Kanotix CpxMini- (230Mb) **quite good** but 250 MB, "toram" boot option runs off ram-disk.

Under 700Mb

 * AntiX (410Mb) MEPIS based
 * Kanotix Lite (551Mb)
 * MEPISLite (626Mb) MEPIS based
 * eLive (662Mb)
 * SAM- (699Mb) SAM Linux is a LiveCD based on PCLinuxOS.