Here's some suggestions in order of their size (as a compressed image rather than install or memory requirements).
Not Linux but worth mentioning
(<2Mb)
KolibriOS (based on
MenuetOS) a GUI OS
(~11Mb) FreeDOS + OpenGEM - DOS compatible GUI
(~23Mb) ReactOS - Windows compatible but still in alpha
(~30Mb)
KolibriOS CD version (based on
MenuetOS) a GUI OS
Under 50Mb
Distros under 50Mb fit onto a MiniCD.
(~3Mb)
Mungkie - (aka 2diskXwin)
(12Mb)
Grey Cat Linux - Slackware based with IceWM
(<15Mb)
SliTaz with JWM
(<50Mb)
DamnSmallLinux - based on Knoppix, but with Fluxbox
(44Mb)
Jinx - with Opera 9, Flash, Textmaker and Planmaker.
(<50Mb)
Austrumi - A business card size bootable live CD Linux distribution based on Slackware.
(<50Mb)
MeanPup -A live distro based on Puppy
(<50Mb)
Mitrax -A live distro based on Slackware
(<50Mb)
Luit - XFCE and ROXFile
Under 128Mb
(<100Mb)
NimbleX sub100 - Slackware based with KDE
(<100Mb)
Puppy Linux - live with IceWM (see also eBoxPup, IcePup, LitePup, MiniPup, PizzaPup, SafePup and XPuppyPro)
(84Mb)
DamnSmallLinux-Not - distro using Knoppix hardware recognition with Abiword and Gnumeric.
(<128Mb)
DeLi - Desktop Light
(<128Mb)
SLAX Popcorn Edition- A smaller version of SLAX with Xfce
(<128Mb)
FeatherLinux - Knoppix based distro with Fluxbox
Under 256Mb
(177Mb)
TinyMe - based on pcLinuxOS, but with Openbox
(182Mb)
Kurumin 6 Light - Knoppix based, with KDE
(~200Mb)
PUD - based on Ubuntu
(~200Mb)
Slax - Distro based on Slackware using KDE
(223Mb)
TinyFlux - based on TinyMe, with Fluxbox
(230Mb)
Kanotix CpxMini, Kanotix, but with Fluxbox
(241Mb) Wolvix Cub
(<250Mb)
Featherweight- based on Feather Linux, but with KDE
(<256Mb)
Flash Linux- Gentoo-based, initially designed to run off of a 256Mb USB key.
Under 700Mb
Distros under 700Mb fit onto most CD-Rs. These should still run in 128Mb RAM on a Pentium III level machine.
(278Mb)
AliXe - Slax based with Xfce
(300Mb) Fluxbuntu - Ubuntu 7.10 based with Fluxbox
(410Mb)
AntiX - MEPIS based with Fluxbox
(551Mb)
Kanotix Lite with KDE
(614Mb) SaxenOS 1 - with EDE
(626Mb)
MEPISLite - MEPIS based with KDE
(662Mb)
eLive - Debian based with Enlightenment
(699Mb)
SAM - A LiveCD based on PCLinuxOS, but with Xfce.
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