Ubuntu will give you the closest experience to Mac OS X - supported for up to 3 years. (They just released a long-term support version.)
Linux Mint is Ubuntu (completely compatible) with some added refinement. It gets very good reviews. I've not tried it yet though.
Opensuse is quite nice. I don't remember how long each release is supported for, but it's a decent amount of time.
Mandriva is okay, but I haven't spent much time with it.
Fedora is good, but often things don't work right. (They use all the newest stuff, which sometimes is good and sometimes is not.) Each release is only supported for one year. (I actually like Fedora a lot, but it's not the best for new users yet.)
I would recommend those in that order. There are many others, but these are easy to use and are all popular enough that finding help should not be too difficult.