RedHat / Fedora

• Pros: Widely Used, Commercial and Free Distros, Stable - Yet fairly new software.
• Cons: Commercial Distro is expensive, Not necessarily the most stable or the most bleeding edge.

Suse

• Pros: Novell Support
• Cons: Resonably priced, but not free.

Debian

• Pros: Great number of packages. Can be built extremely stable. Widest variety of hardware platforms. Extensively tested.
• Cons: Doesn't always have the latest packages (see Ubuntu for new packages).

There are literally hundreds if not thousands of distros out there. Go to DistroWatch.com and look on the right hand side of the page to get an idea of the popularity of some distros.