Social+Bookmarking+Sites+to+Use

Delicious: Possibly the most widely-used bookmarking site on the web. Delicious is quite easy to use, has a simple format, and enables you to create a network of other users and get a single feed for all their publicly shared bookmarks.

CiteULike: Specifically for academic research purposes, though it can bookmark any site on the web. You can also export to EndNote + BibTeX using this site.

Connotea: Specifically for academic research purposes, though it can bookmark any site on the web. You can also import/export from EndNote, ProCite + BibTeX using this site.