Ah, teh L1NuX. Yes, I have to admit, I am a linux geek. Hardcore linux hacker. And what distro do I use? Gentoo. Plain and simple, it's the best thing for ppl who want specific things. It's the most stable distro, period. the portage tools are amamzing, and installing things is easiy (alibet, it takes a while to compile them from source...) and I could go on and on.
Now, this being said, I have a G/F who really shoul have been born in the middle ages. She despises tech, and she would love it if we all rode horses, and lived in small villages. (She's cooky, but I love her.) She also admits that she needs a computer, and she learned early on to hate windows. (Spyware == teh sUxz0rZ!!!!11one) So, I put gentoo on there.
I must admit, having her run gentoo seemed retarded on my part, since she knows > 0 about computers... but she picked it up rather fast. It's REALLY scary doing an install for the first time... and I've never even dared to do a stage 1 install (I'm too damned lazy, and impatient). However, I would really recomend it to anyone who wants to either learn a lot, or 'play'. It's not something I'd say you must be l33t to use, but it is challanging.
Oh, and just so that I don't feel like I was ranting about my fav. distro; SuSE is a nice one that I've used for a few years. It's good, it's GUI, and it's hardware support is awesome. Mandrake is ok, but they don't do enough bug testing I think. Mandrake always feels... fragile to me. Hard to explain. It's easy to break. Fedora Core seems pretty good.. I've fallen out of the 'in crowd' though, so I don't know about the latest version.
Recomendations:
1) Gentoo
2) Suse
3) Fedora
4) anything running a 2.6 kernel

Then again you could run:
Linux, running PearPC, emulating MacOSX, runing wine, emulating VMWARE, running windows98SE, running PearPC, running VMWARE, running Mandrake. (And yes, you CAN do it. No, I don't recommend it.)