(1) What is a good (ideally recent) game that would accept new characters, levels, weapons, props, and so on?
(2) Apart from owning the game, what else do I need to get my Maya models into the game so that I can see them in action?
(3) Can you recommend any good websites where I can download the necessary guides and resources?
As always, thanks in advance!

EDIT: I tend to prefer First-Person-Shooter type games although very rarely ever play them... let alone buy them.