In my mind you have RedHat, Novell (Suse), Ubuntuu are the big three that you might expect to see widely used as either a server or client OS. Some of the other popular ones are Slax, Mephis, Debian, Fedora, and a few others are niche distros that many people use because they want to / can customize..
This is just one mans opinion and there is always someone who will pipe up with how great $DISTRO is, but RedHat and Novell have the server presence and Ubuntuu has the desktop presence which covers a huge segment of the market..