There are some funny(thats obvious...as they are meant for fun) easter eggs in GNU/linux. Im listing out some of them ( because I don't know the rest..).
Free the fish
Press 'alt + F2' then type "free the fish" (without quotes) in the dialog and press enter. A fish comes swimming in your desktop.
But the problem is that the fish never goes away. The only way(which I know) is to kill the panel("killall gnome-panel").As gnome cannot live wihtout the panel it restarts it(without the fish!!).
gegls from outer space
Type this in the "run application dialog" (as above). This will start a game named "GEGL killers from outer space"
where a fish(the fish as in the above case) has to fire on cows and kill them which will be attacking the fish.
Gnome-panel properties
Open the gnome-panel properties window. Click the right mouse button 3 times on any of the 2 tabs (General / background ).This opens another new tab named GEGL where a cow comes waving its hand(it has an extra limb) to you.
Require Quarter
In the gnome login screen type Require Quarter in the username field. It acts as if it ignored what you typed. When you login next time,it says "pay me 25 cents to login".
Try the same with "gimme random cursor"
Apt-get moo
Type apt-get moo in the terminal( bash ). Then it displays ascii art of standard gnu-cow.
Aptitude moo (sequence)
One should experience this on his/her own. Type aptitude moo in terminal. Then go for aptitude -v moo...then aptitude -vv moo....Continue this upto 6 (or more) v's.
Starwars in OPenoffice - calc
Type =GAME("StarWars") in A1 and press enter. This opens star wars game.
Update:
I have tried the above stuff in my department lab which runs on ubuntu 8.04 and made the post accordingly .But unfortunately some of them are fixed in the later versions. When I tried them in my laptop (I use 10.04) Require Quarter is not working....Hope the rest are of them not fixed in the coming versions :P