http://citeseer.nj.nec.com/cs

"ResearchIndex is a scientific literature digital library that aims to
improve the dissemination and feedback of scientific literature, and
to provide improvements in functionality, usability, availability,
cost, comprehensiveness, efficiency, and timeliness.

Rather than creating just another digital library, ResearchIndex
provides algorithms, techniques, and software that can be used in
other digital libraries. ResearchIndex indexes Postscript and PDF
research articles on the Web..."

http://citeseer.nj.nec.com/cs

Research index (though most people still call it citeseer) is the
currently best way to find published papers AND TRACK CITATIONS.  It
is so cool, it's in the FAQ twice. (now 3 times).

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All of the AI Memos and Tech Reports from the MIT AI Lab, going back
to 1959, can be found here:
http://www.ai.mit.edu/publications/pubsDB/pubs.html .  Great
historical documents, plus everything new.  Need Winograd's Thesis?
There it is.