This month's MacFormat has a whole article about OS X easter eggs..
Some of those that I can be bothered to type, are below;
FireFox
In the address bar type "about:mozilla" (no quotes) and you'll see a nice extract of biblical text.
Photoshop
Bring up the Layers palette, click on the arrow for the Palette menu, hold down the Option key while you select "Palette Options".. suprise, Merlin T. Wizard is back!!
For another treat, select the Type tool and click on the Toggle Palettes button in the toolbar. This brings up the Character palette. In the Font Family field, type the name of an installed font, followed immediately by francis.
Once the font name is complete each keystroke will generate a warning beep.. IGNORE them and carry on. Now click on the Toggle Palettes button again and you'll see a photo of Francis and some rather sad text!
NeoOffice
Create a new spreadsheet and in any cell enter the formula =game()
If you press Enter the cell should display "say what?".
Enter =GAME("StarWars") and you now have a fully working Space Invaders game
Speech
Firstly, enable speech in system preferences > speech, and say "tell me a joke".. your Mac will tell you a knock-knock joke
Terminal
In Terminal type "say -vGood xxxx xxxx xxxx" (xxx = any text you want). This will make your Mac sing you the phrase to the tune of "Land of Hope and Glory"
Equally, type "say -vBad xxxx xxxx xxxx" (xxx - any text you want). This will make your Mac sing you the phrase to the tune of the "Funeral March"
Other Terminal goodies include typing "open -e/usr-share/calendar/calendar.history", this will make your Mac spit out some interesting dates from history... ORRRR... try adding the .lotr bit at the end, instead of .history (for some Lord of the Rings dates)
Weather Widget
Press Alt+Cmd and click on the image at the top of the weather widget to see what the weather is like in Nowhere.
Keep clicking to cycle between all the different weather pictures if you want.