Besides RHEL there is also SuSE SLES / SLED with apparently 7 years of "general support" + 3 more of "self support" (guess that means you just get serious security fixes).

http://support.novell.com/lifecycle/

In addition to the shorter support time, with Ubuntu LTS it's a bit too easy to install packages from "universe" that aren't actually supported for more than 18 months...