Ubuntu
The famous Ubuntu is a general-purpose distribution based on GNOME. It has a six-month release cycle.
Debian Current
A stable distribution, much beloved by its fans. Fast and reliable.
Debian SID
Slightly less reliable than Debian Current, but stable enough for most users. Also, much more up-to-date.
Kubuntu
Based on Ubuntu, but uses KDE. Also, much faster.
Xubuntu
Based on Ubuntu, but uses XFCE. Very fast.
Ubuntu LTS
"Long-term support" versions of Ubuntu are released every two years, but have longer support and are more stable.
SabayonLinux
A newer distribution based in Italy, SabayonLinux is designed to install a Gentoo-based system in minutes, rather than days.
Fedora Core
Fedora Core was forked from Red Hat Linux in 2003, and is still sponsored by them. It remains fast, powerful, and feature-packed.
Gentoo
Gentoo, aka Enoch Linux, is an incredibly fast distribution that compiles itself from source. Gentoo installs have been known to take days on end.
openSUSE
openSUSE is backed by Novell and Microsoft. (Yes, I said Microsoft...) Powerful, with enterprise-class features.
Arch
Arch installs a base system, optimized for Pentium II and later systems, that you can build on. It's not recommended for beginners.
Mandriva
Mandriva, formerly Mandrake, is a desktop aimed at corporate ex-Windows users that has decent performance and lots of extra features that some other distributions lack.
PCLinuxOS
A fork of Mandriva designed to appeal more to slightly-more-experienced users. A power-user's distribution.
Frugalware
Frugalware is a cross between Slackware and Arch Linux- it follows Slackware's design concept, and uses Arch's faster packaging system.
Ulteo
Based on Kubuntu, and started by the founder of Mandriva, Ulteo is a self-maintaining online desktop that can be run within a browser via Java.
KateOS
An XFCE-based gaming system with a myriad of themes and powerful, built-in multitasking abilities.
Damn Small
"50MB of penguin power": a tiny desktop that fits onto the smallest of CDs, yet has everything most other distributions do. Great for gaming.
Freespire
A Windows-like desktop with a powerful program-installation system and a wide array of non-free add-ons, like MP3 playback and video drivers.
MEPIS
Based on Debian, but with numerous useful extra features and a more newbie-friendly approach. Also, the first distribution to have a live CD with a graphical installer.
gNewSense
Based on Ubuntu, but completely open-source.
Pardus
An up-and-coming source based distribution. Pardus comes with 3.4 gigabytes on software on one CD, all of which runs faster than normal, thanks to several unique features.
FreeBSD-Release
The most popular type of BSD Unix. Not really a Linux distribution, but fast and incredibly stable.
FreeBSD-Stable
FreeBSD releases are few and far-between. Stable enough for desktop use, this repository helps you keep current.
PC-BSD
PC-BSD is based on FreeBSD, and comes with everything you need for a FreeBSD desktop.
SAM Linux
SAM Linux is a fork of PCLinuxOS that uses XFCE to be even faster that normal.
Wolvix
A live CD based on SLAX Linux. Includes XFCE and other nifty features, such as NVidia drivers. Fast and flexible!
OLPC
A CD designed for educational purposes. Part of the $100 Laptop project, and will be the OS that runs on $100 Laptops. Based on Fedora, and heavily modified.
Foresight
The complete GNOME project, i686 (Pentium II) optimized and designed to "just work". No more, no less.
Myah OS
If you want a small desktop that runs as fast as possible, Myah, an i686-optimized XFCE-based installable live CD, is worth a look.
Linux Mint
Based on Ubuntu, Mint comes with a more refined out-of-the-box program selection, along with some extra configuration utilities.