Today I reinstalled GNOME from scratch on my machine due to a mistake I made yesterday (which blew away half of it). After starting it, I saw a problem I had never noticed before. I have been able to solve it, although I haven't figured which is the root cause yet. Anyway, while looking for where the problem was, I discovered three easter eggs in the GNOME Panel module. If you don't like spoilers, stop reading now.
Two of them are exactly the same in the results they produce, although they are accomplished in completely different ways. The first one is to open the "About Panels" dialog (right-clicking on any visible panel) and, when the about dialog pops up, press the 'f' key three times. The second one is to open the "Run Application..." dialog, type the "free the fish" string and click "Run". After that, you will see a fish appear on screen, which will keep moving from left to right (clicking on it changes its direction). Unfortunately, it seems that the only way to stop it is killing the panel process).
The last easter egg is achieved in a similar way to the previous one, but the results are more spectacular. You should type the "gegls from outer space" in the "Run Application..." dialog and then click "Run". As a result, you will get an "Space invaders" game clone which uses cows rather than aliens.
Wow! These are the first easter eggs I discover by myself :-)