This post is only to give a useful link: Quick Reference Cards. You will find there a lot of information on programming languages, operating systems, editors… The interesting thing about these cards is their format. Each card is a one (or two) page(s) giving a list of the most important commands, shortcuts or syntax on a subject.
Another site like this one is refcards.com. If you know other sites with reference cards, don’t hesitate to leave the link in a comment.
