CentOS 5.2 - Send in the Clones
CentOS, for those unfamiliar, is a clone distribution. The maintainers take the freely-available source code released by Redhat for its commercial Redhat Enterprise Linux (RHEL) product and recompile it, stripping out any trademarked artwork, then redistribute it as CentOS.
Because of this, CentOS is probably the closest equivalent to a free-of-charge RHEL that you can get. I recommend CentOS to anyone trying to learn more about RHEL, particularly for Redhat certification exams or corporate reasons.
Because it's based on the stable-by-design RHEL rather than Fedora, each version of CentOS focuses more on evolution than revolution. You won't see the eye-candy or new, largely untested gizmos that make Fedora popular.
The end result makes CentOS stable, but not particularly thrilling. I personally don't mind this (having used stable-but-not-exciting Ubuntu Hardy for several months now), but I thought it best to warn you ahead of time.