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/