I'd agree with most of what Clegg is saying. CENTOS is not technically a fork of RHEL; rather, Red Hat releases the sources for RHEL, which the CENTOS people then compile (without the Red Hat branding). CENTOS aims to be binary-compatible with RHEL, so if you want RHEL's good reputation and stability without paying loads for support, CENTOS is a great option. Additionally, the BSDs make excellent servers, but if you're familiar with Linux then they will be a bit different.
I'm a Debian user, so you'd think I'd be recommending its use. Don't get me wrong, it's very good, but it's also not supported for very long. Ubuntu's latest LTS, released this summer, will be supported until June 2013 on the server. Debian's upcoming release, lenny, will only be supported for one year after their next release--and since Debian releases are aiming for an 18- to 20-month release cycle, you're looking at about 2.5 years of support. So, yes, Debian is certainly more stable, but if you want to keep the server around for a long time, Ubuntu might be a better choice.