A very useful site for programmers is the Quick Reference Site, maintained by Tim Sinaeve. This site features:
- Quick Reference Cards, short sheets cointaining the main commands for e.g. utils like Subversion or vi and so, and programming languages. The most valuable resource, although some cards can have 8 pages!
- E-books, that range from manuals to learning and reference books.
- Papers and tutorials (self described).