hold apple key, option (alt), and control, and press 8 to change the screen colours negative. useful for reading text that contrasts poorly with the background, like green on black.

hold control and use the scroll wheel to zoom in/out on the cursor

apple key, shift, and 4 will let you take a screen shot, but only of the area you select when the crosshairs come up

if it hasn't already been mentioned, logging in with username 'root' plus your normal password will give you access to all files on the comp