arXiv.org: Over 383,443
(and growing daily) free papers in Physics, Mathematics, Computer
Science and Quantitative Biology.
This is an excellent source of (almost always) reliable information
about an incredibly wide variety of topics. Wikipedia often has
very clear, not too technical explanations technical terms.
Lots of good information here, often at rather technical level.
Networks and Graphs
M.E.J Newman, Models of the Small World: A Review. http://arxiv.org/abs/cond-mat/0001118/.
Published in: J. Stat. Phys. 101, 819-841 (2000). A short review
article about empirical results and different models of small world
M.E.J Newman, The structure and function of complex networks. http://arxiv.org/abs/cond-mat/0303516/.
Published in SIAM Review 45, 167-256 (2003). An excellent, clear,
and thorough review article about complex networks.