A message from your friendly neighborhood arcanaloth
Mithran said:
Hi I'm going to be joining a plane hopping game here soon and I'd like some information/suggestions on character concepts since I've never played a game set in the planes.
If it's basic information about the planes, you can't go wrong with the 3e Manual of the Planes. Or if you want to look at the original sources that it drew upon, and which go into much more detail and depth, look at any of the 2e Planescape material. There's also the 3.5 Planar Handbook, which is more player oriented, but has mixed reviews (has good stuff and lame stuff. It's watered down Planescape).
There's also
Planewalker the 3e Planescape site. We've got tons of information, character ideas, races, items, spells, plot hooks, information on specific sites in the planes, planar factions, NPCs, stories, artwork, etc. Everything you might need, and it's all free.
We've been updating the 2e Planescape material to 3rd edition. For the 2e version that does much of what we're doing, there's always mimir.net as well. It's a yummy little idea farm.