2d Collision Detection as Done in the game "N"

This is not a new article by any means, but it's a good overview of the kinds of approaches one might take in collision.

One of the directors at my workplace has the motto "Collision is gameplay" because of the high importance of getting these dynamics right -- whether you allow things to interpenetrate or not, how general the allowed shapes are, etc. The algorithms can get complex very quickly if you let them, and sometimes an accurate and realistic physical simulation turns out to be the wrong thing.