I'm a sucker for cheat sheets, those all-in-one reference guides illustrating the most useful commands, shortcuts, terms, and other tricks that will save you time at the keyboard. These two sites are the motherload of cheat sheets.
The first is Manual.su's appropriately named Cheat-Sheets.org. From .NET and ActionScript to Zen Coding, you can find and download PDFs or other files for likely any program, system, or computer language you're interested in learning or want to get to know better. There's even a handful of Wikipedia reference cards/cheat sheets and links to things like a gift ideas index and collection of freeware.
If Cheat-Sheets.org doesn't have what you're looking for, you might try TechCheatSheets.com. The domain name says it all. You'll find cheat sheets for PowerPoint keyboard shortcuts (as a tri-fold brochure, no less), Adobe Photoshop, Dreamweaver, and Illustrator keyboard shortcuts, and over 150 other cheat sheets there.
Enjoy! If you have a favorite cheat sheets resource, let us all know.