Many of you may have come across this site, but I recently discovered its worth when I was searching for a study document for a course. Man this site really has a lot of documents, Documents on any subject you may need, it is there. The search functionality is quite good and returns relevant results to your search.
The best part is that you can download documents in pdf format, embed a document in your blog. You can follow recent updates from scribd on their blog site http://blog.scribd.com/ . Apart from their search one could browse through a vast category of books if you don't have any particular subject in mind. Simply a great online tool to learn and spread the knowledge around by uploading your documents online.
Of course you can also buy ebooks apart from the free ones they have. If you didnt know about this site, do check it out.