The ATTW list (attw-l) is currently discussing online citation software. Here are three:

CiteULike
KnightCite
CitationMachine

Of the three, CiteULike seems to be the best for sharing citations. It tags these (like del.icio.us or Flickr) and lets you download them in EndNote or Flickr format. The other two are more geared for turning individual pieces of information into correctly formatted citations. They might be ideal for using in a first-year comp class.