Thor grossed $437 million worldwide, and Marvel Studios has a public mandate to release two movies a year. 

In 2011 it was Thor and the upcoming Captain America: The First Avenger

In 2013 it will be the superhero team-up The Avengers with no second release yet announced.

In 2013 we’ll get Iron Man 3 on May 3rd and Thor 2 later that Summer.

Walt Disney Pictures confirms.
The Avengers 2 is in the works as well as a Nick Fury film. 

At the moment we have the following either released, pre-production, or planned:

The Incredible Hulk
Iron Man
Iron Man 2
Iron Man 3
Thor 2
Captain America
Captain America 2
The Avengers
The Avengers 2
Nick Fury

There are also talks about a HawkEye, Hulk 2, and some other minor character spinoffs.