Scribd is a extremely useful file-sharing site. By files I mean, documents, pdfs, slideshows, books etc. In case you are looking for some academic material, then it's a great first stop.
I was looking for material for a test that I am preparing, and certainly found a lot of useful stuff there.
It is very high on usability factor; I like the way it indicates download progress for large files, and also page number indication (such simple trick, but never used before) while scrolling pages of documents.