RedHat Linux, but designed for corporate scenarios. Released less often than RedHatLinux, kept more stable, supported to the Nth degree. If someone has certified their software for Linux, it's probably going to be on this.
http://www.redhat.com/software/rhel/
Red Hat Enterprise Linux products are delivered through an annual subscription with one year of support services and access to Red Hat Enterprise Network.
Comes in three flavours: AS (Advanced Server) for 4+ procs & clustering, ES (Enterprise Server) for 1-2 proc servers (this is probably the one you want for most cases), and WS (Workstation).
Is not available free, but is Free. (Yes, the GPL allows this.) If you get the CDs from someone with a subscription, and build the SRPMS yourself, it would seem you're allowed to have it.
Highlights
Latest release as of 2004 is 3.0.
An indepth discussion of what you can and cannot do with Red Hat Enterprise Linux lead to a number of "free reimplementations" of the 100% OpenSource RHEL; see RedHatRebuilds.