Gnome Easter Eggs!

• Hit the Alt-F2 keys together, 'Run Applications' dialogue will appear. Type gegls from outer space and press ENTER. You will get a game.
• Type free the fish. Wanda, the fish will appear on your desktop. If you click her, she will swim off the screen, but will come back later. To remove Wanda from desktop, type killall gnome-panel to a terminal. Your gnome-panels will refresh and Wanda will disappear.

Via ArsGeek