After the credits roll on The Avengers, there is a brief scene that appears to hint at who the next villain will be. I thought it looked kind of like the Red Skull, but others I saw it with disagreed.
Who was it?
I just got back from the film, and the mid-credits scene villain is indeed Thanos. He's a cosmic-level villain who seeks to control the universe. The idea that the entire film's plot was just a scheme his minions were working on for him is a pretty great introduction for him. His desire for objects of power suggests that his goal was to get the "Tesseract", which in the comics are called Cosmic Cubes.
In addition, his Chitauri minion referred to attacking Earth as 'courting Death', which is a reference to Thanos having a thing for Death.
The guy in the scene after the credits, is The Mad Titan, THANOS. One of the most powerful beings (villains) in the Marvel Universe.