Here's another one i ran into recently - Project Gutenberg, the oldest producer of free electronic books (also popular as eBooks) on the internet. Now, there's this group called Project Gutenbergs Distributed Proofreaders. Founded in 2000, this group supports digitization of publicly avaialable books - which means they take public books and convert them into electronic form, which can be used over the internet. And then such digitized books are archived into the Gutenberg project. Books are avialable in several formats like PDF, HTML, Audio books, CD images, PDA versions and what not..
A recent post at slashdot, anouncing that this group has posted it 5,000th ebook to the project brought it to my attention. U can find several immortal works here (e.g: Works of Jules Verne, Aeosops Fables, Archimedes, Aristotle and more ..). Here's a link to the catalog