links to knowledge sharing or knowledge management site; larger site is most comprehensive listing of web2.0 sites I have seen, organized into many categories; blogger is from Netherlands
Firefox extension offers an addictive way to search via categories or keywords or happenstance; you train it, with thumbs up or down on sites, so it learns your tastes; I prefer searches on keywords so far, and have had little desire to "network"
CiteULike is a free service to help academics to share, store, and organise the academic papers they are reading. When you see a paper on the web that interests you, you can click one button and have it added to your personal library.