I have been searching for sources of scientific literature for some time now, and I have had only minor success. I was looking for a database that has a large amount of scientific papers. The hard numbers and detailed graphs kind, not the story about a recent developement, such as those found on newscientist.

Of the sites I've found, the best I know of is http://arxiv.org/ It has many papers from many fields. I have also found http://www.sciencedirect.com/ which, while having a large variety of papers, have few in total to offer, and focuses more on advertising differant Journals.

Specifically, I'm looking for free sources. While I would love to browse the archives of Nature (the journal) I don't make all that much money, and what I do make is needed for other things besides supporting a hobby of reading advanced papers.

If you know of another large body of scientific literature, please post it. Accessibility of knowledge is very important.