The Citeseer Scientific Digital Library contains downloads for many excellent papers: http://citeseer.nj.nec.com/
ACM provides some free papers, but IIRC to view many of them you must be a member: http://www.acm.org/
From RFCs (for Internet protocols) to USENET newsgroup FAQs: http://www.faqs.org/
ARPANET papers which are useful if you want to know about computing history (especially the Internet): http://www.archive.org/texts/arpanet.php
USENIX has many research papers, and I believe most of them are free to view: http://www.usenix.org/
ro If you know the title of the paper you're looking for, then enter it into google with '+ pdf'. If its at all popular, its out there.
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