Here's some suggestions in order of their size (as a compressed image rather than install or memory requirements according to free -m). Most run on x86 hardware and are graphical.
Not Linux but worth mentioning
(<2Mb) QNX Floppy demo
(<2Mb) KolibriOS (based on MenuetOS) a GUI OS
(<2Mb) Balder (based on FreeDOS)
(~11Mb) FreeDOS + OpenGEM (4.4Mb) - DOS compatible GUI
(~23Mb) ReactOS - Windows compatible but still in alpha
(~30Mb) KolibriOS CD version (based on MenuetOS) a GUI OS
(<50Mb) DamnSmall BSD - based on FreeBSD, but with Fluxbox
(<100Mb) Milax - Solaris live CD
INX Command line Ubuntu designed for usability.
Distros under 50Mb fit onto a MiniCD or sometimes floppy disks (usually 1.44Mb each). Best in category: Damn Small Linux.
(~3Mb) BasicLinux - Slackware 4 + X + e-mail + presentation
(~3Mb) Mungkie - (aka 2diskXwin)
(~4Mb) Blueflops - 2 floppy graphical web browser
(8.9Mb) Geexbox (Media Center only)
(10Mb) Tiny Core - Linux 2.6 kernel, Busybox, Tiny X, Fltk, and Jwm
(12Mb) Grey Cat Linux - Slackware based with IceWM
(~25Mb) SliTaz with JWM
(29Mb) LinuxConsole Micro - GUI IceWM
(<50Mb) DamnSmallLinux - based on Knoppix, but with JWM and 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
Best in category: Puppy Linux.
(80Mb) LiMP - Linux Media Player turns your PC into a jukebox.
(<100Mb) NimbleX sub100 - Slackware based with KDE (or 69Mb with no GTK or multimedia)
(<100Mb) Puppy Linux - live with JWM (see also eBoxPup, IcePup, LitePup, MiniPup, PizzaPup, SafePup, SurfPup and XPuppyPro)
(82Mb) KioskCD - web browser only
(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
Best in category: Ubuntu GTK1.2 Remix.
(177Mb) TinyMe - based on pcLinuxOS, but with Openbox
(182Mb) Kurumin 6 Light - Knoppix based, with KDE
(183Mb) WebConverger Mini based on Debian Live
(187Mb) Ubuntu GTK1.2 Remix - Ubuntu Hardy Heron for sub 300Mhz PCs
(200Mb) NimbleX - Slackware based with Enlightenment, IceWM, JWM, Fluxbox
(~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.
Distros under 700Mb fit onto most CD-Rs. These should still run in 128Mb RAM on a Pentium III level machine. Best in category: Crunchbang.
(278Mb) AliXe - Slax based with Xfce
(273Mb) Icebuntu - Ubuntu + IceWM (live cd username is ubuntu)
(293Mb) Draco Linux - SLiM login manager + Fluxbox + Xfce
(300Mb) Fluxbuntu - Ubuntu 7.10 based with Fluxbox
(319Mb) WebConverger Maxi based on Debian live
(410Mb) AntiX - MEPIS based with Fluxbox
(440Mb) TheLittleDebian - Spanish
(551Mb) Kanotix Lite with KDE
(614Mb) SaxenOS 1 - with EDE
(626Mb) MEPISLite - MEPIS based with KDE
(633Mb) Crunchbang - Ubuntu with Openbox (and Conky!)
(662Mb) eLive - Debian based with Enlightenment
(673Mb) SOAD - OpenSUSE 11 with Enlightenment 17
(693Mb) MoonOS - Debian with Enlightenment 17 or LXDE
(696Mb) OzOS - Xubuntu (or Debian) + e17
(699Mb) SAM - A LiveCD based on PCLinuxOS, but with Xfce.
Ubuntu minimal install
Debian minimal install
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