Slackware, the Linux that spread the word to early adopters.
Debian, the first community Linux.
The openSUSE (Novell) project gives Linux developers and enthusiasts everything they need to get started with Linux.
Gentoo has extreme performance, configurability and a top-notch user and developer community.
Red Hat, the Linux that actually put Linux in the enterprise.
Ubuntu, the Linux that showed that it was an operating system for everyone.
We'll add one more. It's actually a variation on the Debian build mentioned above.
There is one group from the Royal National College for the Blind, the UK's leading college of further education and training for people (16+) who are blind or partially sighted and with other disabilities, that is building a special distribution aimed at the Visually Impaired user. It's called Vinux.
Vinux is a Linux distribution based on the Debian distribution, designed especially for visually impaired users. Vinux provides a screen-reader, full screen magnification and support for Braille displays out of the box. It can be run from the Live CD without making any changes to your hard drive. If you like it, you can install it to a USB thumbdrive or to your hard drive either alongside Windows or as a complete replacement for it. There is also a virtual version available which can run Vinux as a guest operating system using VMWare Player on Windows. The main screen-reader/magnifier is called Orca and the Braille display is supported by Brltty. There is also a secondary screen-reader installed called Speakup, which is a console based screen-reader. This is aimed at more advanced users but can also provide a useful backup in the event of any problems. Vinux provides a wide range of open source software including an internet browser, a file manager, a text editor and various multimedia applications. Vinux is designed to be a completely secure modern desktop system for all the family, visually impaired and sighted alike.
They are looking for people to install and test this on-going project. You could use this opportunity to learn one type of Linux and still keep your other operating system for your normal work and help create and fine-tune a fully accessible Linux build for the world to benefit from.