Physics Body Editor (previously known as box2d-editor) lets you import your game images and easily define collision outlines for them. These outlines are automatically transformed into multiple convex polygons to fit physics engines requirements. Then, an in-game loader can be used to retrieve the collision data from the saved files and build your bodies in just with as few lines of code as possible.
The editor is engine-independent. Therefore, it may be compatible with any physics engine, like Box2D, Chipmunk, Farseer, etc. All you have to do is to write a custom loader for your preferred engine implementation if there is none already provided.
NOTE: if you indeed write a custom loader, I would love you to share it with other people. Send it to me at aurelien.ribon at gmail.com and I'll include it directly in the distributed zip file, with credits given to you. Thanks in advance.
Reference manual: http://www.aurelienribon.com/blog/projects/physics-body-editor/