If you want to do it for roleplaying, rather than rules, then you might want to try (and I know that there have been many suggestions already, but I'll add another one anyway :P) Ars Magica. As the name may suggest, it's primarily focussed on wizards ('magi') and is set in 'Mythic Europe': basically medieval Europe as people believed it was at the time. It's very much focussed on roleplaying, and rewards creativity, rather than limiting options so it can make sure that everything is equally [not] powerful. As the core rulebook says 'Magi in Ars Magica aren’t artificially weakened for the sake of arbitrary “character balance.”' Uniquely, as far as I know, each player is encouraged to create two characters for them to play simultaneously: a magus and a non caster.

It also suggests and gives advice for not having a single dm, but sharing the dming between players for different adventures, which is again unique AFAIK.

Oh, and the fourth edition is also free, The latest edition (fifth), I hear, fixed a number of the issues with its predecessor and is generally considered an improvement, but 4th is perfectly playable if you rethink the combat rules a bit*.

*As it is, the best chance of winning combat is when you enter naked wielding a stick!