Interesting Library / Book Sites

While surfing the net I came across a Stumble Upon post that lists the following interesting library sites:

Bookcrossing -- The World's Biggest Free Book Club
Forbidden Library -- lists books that have sometime in their history been banned or restricted.
Utopia Library -- collection of texts and interviews exploring possibilites and boundaries of social evolution
Poetry Archive -- Poetry, poetry, poetry
Planet PDF -- Books that are in PDF form
Library Thing -- Cataloging one's books online

For a list of all libraries on the web, BookYards alphabetical list is here, and categorized according to specialty here.