I always forget these, so I’m putting them here where I know I’ll be able to find them again:
Command-Option-Control-Shift-4: copy the crosshaired selection to memory.
Shift-Command-3: shoot the whole screen (both if you’re running a double display setup like I do at work)
Shift-Command-4: gives you crosshairs to select a chunk of the screen for the screenshot.
Also, with a third-party preferences pane like OrangePrefs (can’t find a link for it right now) or by using Terminal to get into the guts, you can set the screenshot format to various different file formats: TIFF (the default), JPEG, and PNG.