Any free unix is probably a good choice. BSD and Linux both come in different flavors.
If you want RedHat but without having to install all the latest patches, take a look at KRUD (Kevin's RedHat Uber Distro) which is the latest RH with extras and all the security patches added in.
Debian is nice for the more technically minded, and Mandrake is easy to install and maintain for beginners, but is pretty capable as a server at the same time. Slackware is great if you really like doing stuff yourself with no oddball gui admin interface in the way.
BSD is nice, but doesn't scale as well on multi-CPU machines as linux. Generally Linux is faster under multi-process load, while BSD is faster under heavy network load. They're both good unix/unixlike oses.
The latest versions of Solaris for Intel are looking more and more like a linux distro with Gnome and other Gnu packages showing up in force.
Of course, if you have enough drive space, you can install more than one distro and play with them in turns.