SciVee has been around for a while, and is still cool: it's the YouTube of academic research. How it works is simple: you upload your presentations and research videos to the site so others can see and comment on your work. This site is not only a great place to learn about research in a range of scientific disciplines, but amounts to a kind of 24/7 virtual conference.
academia.edu is a bit like FaceBook, allowing academics to create networks of like-minded (you guessed it) academics. But whereas FaceBook links people by keyword common interests, Academia.edu links you by fields of specialization—so everyone tagged as interested in "rhetoric" is virtually linked. Another feature of the site is that it creates visual hierarchies of departments, so images like this can be filled out by members online:
Mendeley is one I came across this week, and it's different from the others in that it involves a local application that you download to your desktop. The local app allows you to store, sort, annotate, and organize PDFs and other research files; the app also interfaces with the website (www.mendeley.com) to allow you to introduce your materials into the social network. Oh, and did I mention that it generates the things all academics love, bibliographies? A recent review of Mendeley can be found here.
A few others include:
citeulike.org: social bibliographic site
delicious.com: social bookmarking site
connotea: reference management app
bibsonomy: social bookmarking