Some very useful keystrokes you might not know about. (This version is for OSX 10.6, Snow Leopard.)

Control-click brings up a context menu: this is the same as right-click on a two-button mouse.

Control-click (or right click) on a word in some apps and choose "look up in Dictionary."

Hit F9 to see thumbnails of all windows, F11 to hide all applications and see the desktop.

Hit F12 for Dashboard. F12 again hides it. Configure widgets by clicking on the i. Add widgets by clicking the + and selecting a widget. You can find other widgets at apple.com.

Quick look. In finder, select a file or group of files and hit the space bar, and you'll get a popup window with a preview.

Screen shot. Cmd-shift-4 gets cross hairs, hold mouse, Drag, release. Picture is stored on Desktop.

To put the computer to sleep: cmd-option-eject.

How to force a shutdown if the computer won't shut down normally (hold power key 10 seconds).

Hit cmd-space for a Spotlight search.