Scientific Linux -- Like Red Hat ... but more scientific (and free)
Scientific Linux has quite a pedigree, according to DesktopLinux.com. Developed by Fermilab, CERN and other heavy-science types, the distro is based on Red Hat Enterprise Linux recompiled from source (which must be available according to the GPL license under which Linux is released) and is said to be mostly compatible with RHEL.
I've wanted to try CentOS, the other free "remix" of RHEL, but I would rather try a live CD first. Scientific Linux has it, and I'm downloading the ISO now. I'll report later on how well it works.