Cent OS, a free version of Red Hat Enterprise. In my honest opinion Red Hat has sucked ever since the end of Red Hat 9 and I will never go back. Further more if you are looking for an easy server to manage and has a lot of 3rd party support then go with Debian for your server needs, and or Ubuntu though very close Ubuntu may be easier for a guy less experienced with linux to play with.
Overall if you do not mind putting in extra time and doing things right FreeBSD is the way to go. As said earlier it is not just a kernel, the Operating System is more complete and honestly I think you have a lot more in depth control.
Just my
