These are some math-related links, with no particular order within each section.
CiteUlike is a free service for organizing and sharing bibliographic references. It is useful to keep track of papers you read and automatically store all references in BibTeX format.
ProofWiki is a wiki for theorems and proofs. The idea is to gather known results in an organized way, so that it can serve as a repository of complete proofs. It is still beginning, but I like the concept a lot: it could grow as Wikipedia and evolve to be a wonderful reference of mathematical knowledge.
"Sage is a free open-source mathematics software system licensed under the GPL. It combines the power of many existing open-source packages into a common Python-based interface." And, you can use Sage directly online.