The Advanced Book Exchange. Free service for those searching for books and manuscripts on the Internet. ABE has over 1.4 million rare and out-of-print books in its database.
Blackwell's of London. Quick and easy search engine for books, and
shipping is surprisingly fast and reasonable [about 3 weeks for overseas,
and about $2.50 for shipping.] Some good deals on Teubners and most
paperbacks.
The Tech Archives at M.I.T. Collection of translated classical
texts from M.I.T.
Translations are usually old, but still useful to do word and concept
searches.