CiteULike and more

CiteULike is in my opinion probably one of the finest web based tools for doing research. We even have our own group page there, in which you'll find a very broad scope of interests. Another one of my favourites is Diigo, which is a best-of-breed combination of Google Notebook and del.icio.us. It does much the same things as del.icio.us, but with this tool you can also highlight and annotate parts of documents found on the web and keep notes regarding those snippets. These will be stored, such that when you visit the page again, the highlighted sections will immediately stand out. Using their browser plugin/toolbar, you can even simultaneously add links and tags to other tagging software (Simpy, Furl, del.icio.us, Yahoo!, and Connotea to name just a few) and automatically create links in your own local bookmarks folder.