CustomGuide has guides for popular Microsoft, Apple, and Adobe software. Most entries have both present and past versions of the application in question. The entry for Outlook, for instance, has quick-reference sheets for Outlook 98 through Outlook 2007.
Each guide includes a diagram of the main interface, keyboard shortcuts, summaries of the menus and the tools found in them, and other relevant tips for using the application more efficiently.
Know of a good resource for software guides and cheat sheets? Let's hear about it in the comments. The guides are free and in PDF format.