As most people who know me know, I am an avid role-player and have been since the early days of D&D. As such, I see shows like Avatar (in all its incarnations) as a great way to integrate new concepts into the game; nations where magic (i.e. "bending") is seen as a normal, everyday thing by the populace and nations defined by the nature of their magic. The d20 OGL (also known as D&D 3.x) has allowed many aspiring gamecrafters the ability to interpret "bending" to D&D/d20, but often with less-than satisfactory results. This is through no fault of their own; ever since the days of 1st edition AD&D there have been elementalist wizards of several different flavours (Wu Jen from Oriental Adventures, Elementalists from Al-Qadim), enhanced with cleric/druid portfolios relating to the elements; the problem is the very nature of D&D magic: it simply is not geared towards an "all-single-element" source of magic. I've found some degree of success in using the Wheel of Time Weaves relating to the elements, but it's still not quite the same and the homebrew systems found online are either massively overpowered or just too complex for use.
So what to do?
Depends on just how closely you want to emulate the Avatar setting; what is more important? The interaction between benders/non-benders and benders of different sorts? or determining just how much water can be shoveled over with a third level technique?
I'm sure that there are strong arguments for both sides, but to me, it is more important to get the feel right; Fire Nation Evokers, Air Nomad Conjurers (the specialization includes teleportation, dontcha know), Earth Nation Transmuters... Or maybe using the new version of Oriental Adventures for Air Nomad Shamans, Fire Nation Wu Jen, Water Nation Clerics... Necromancer "bloodbenders", druidic "swampbenders"... so many possibilities.
A wiki like this one is great for this purpose; a free, detailed sourcebook with no need to deal with ill-advised house rules that turn your typical firebender into a nuclear-powered fireball-tossing machine.
Does anybody play? How do you play?