If you want a pretty good cheap Unix dist. you can get Solaris 10 for free from Sun.com the download page is
http://www.sun.com/software/solaris/get.jsp for Solaris 10, some older ones are also out there. That is the best free Unix that I know of.
As far as Linux goes there are a lot of good beginner distrobutions. While something like Fedora would probably be the easiest for a beginner to use because of it's extensive gui interface and autoconfiguration, I would ot recomend it for the beginner. I highly recomend something like Basic Linux:
http://www.volny.cz/basiclinux/. The reason I think this is awsome is because it is simple enough and well documented enough for a beginner to figure out, yet there is no gui to beginwith, so one is forced to poke around and learn the system. If you know enough to get the gui up on it, then you know you've earned it.