Spore developers, like many game developers, have included Easter Eggs into the game. Here are the ones we know about (add more if you discover them).

Spore Creature Creator Easter Egg

If you spin the galaxy really fast, near the main menu, the menu buttons for "Load Creature", etc. disappear, and Will Wright's head will appear:


Spore Developer Heads Easter Egg
In the main menu, where you can start a new game there is a galaxy that can be spun around by clicking/dragging it in which ever direction you want. Once the galaxy is spun around fast enough, pictures of what appears to be developers of the game spiral out of the galaxy.