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/
For RFCs specifically: http://www.rfc-editor.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/
arXiv (http://www.arxiv.org/ ), formerly known as xxx.lanl.gov, is a huge preprint archive. It started as a physics preprint archive, but it now covers also mathematics and computer science.
PhysNet (http://www.physnet.net ), Physics' departments and documents
HAL (http://hal.ccsd.cnrs.fr ) is a french server for depositing scientific papers, it covers physics and other disciplines available from arXiv and has a write through to arXiv for those papers. But it is open for any scientific discipline.
ro If you know the title of the paper you're looking for, then enter it into google with '+ pdf'. If it's at all popular, it's out there.
VL It will also be on the internet if the author is computer savvy enough and interested in 'publishing' it, and yes I use Google. If you don't know the name of the paper, go to a university library site; they tend to have subscriptions to abstract databases and such that are free to search.