If you have absolutely no clue, then you are probably best off booting Ubuntu from the CD and trying it out before installing.
Other than that..
There are zillions of Linux distributions: (K/X)Ubuntu, openSuSE, Fedora, Mandriva, Debian, Gentoo, Slackware etc etc etc
Four BSD versions: FreeBSD, NetBSD, OpenBSD, DragonflyBSD
The PC Unix: Solaris
All sorts of weirder OS: BeOS (almost dead), QNX, Minix and a whole bunch of really exotic (read: unusable) ones
Getting OS X to run on a usual PC is Voodoo (and pretty much illegal for a normal person)