I came across CiteULike on Seb's Open Research blog a week ago, tried it out, and it seems very useful. Basically it's a cross between bibliography management software like JafRef, and social bookmarking like del.icio.us. You can use it to keep track of the papers you find, and export in the BibTex format. Then you can see what other people added given paper, and see what papers they are reading. You can also add comments to papers and view other people's comments (not many at this point). Suresh has a more detailed review.