Probably most widely used distribution and Red Hat is pretty involved in getting Linux on the desktop...

Latest Version: 3.6
Home Site: http://www.cdrom.com/titles/os/slackwar.htm
Download Site: ftp://sunsite.unc.edu/pub/Linux/distributions/slackware/

Slackware is second on the popularity list. Some consider it to run a tighter ship than Red Hat.

Latest Version: 2.0
Home Site: http://www.debian.org
Download Site: ftp://sunsite.unc.edu/pub/Linux/distributions/debian/

Debian is no longer backed by any specific commercial backer, it is totally ran by a group of volunteers.

Latest Version: 1.3
Home Site: http://www.caldera.com

Caldera is designed pretty much for corporate use in mind