Desktop Distros:

• Fedora (from RedHat), OpenSuSE, Ubuntu (based on Debian);
• all completely free (in all senses);
• updated frequently; end-of-life approx 18 months.

Server Distros

• Some cost money, others are completely free;
• RedHat Enterprise (Scientific Linux, CentOS)
• SuSE Linux Enterprise Server (aka SLES) — UoM site licence.
• updated infrequently (except security); end-of-life 5–7 years.

Geek Distros

• Debian, Gentoo. . .   —not for beginners!

More:

• And many more. . .