Depends on what you like and what you like to do!
If you are iTunes oriented, I love the Amazon Album Art Widget for Dashboard - very fast.
If you like inter-stellar travel, Celestia is pretty neat. If you prefer to stay firmly rooted on earth, Stellarium is nice.
If you like to play, LEGO Digital Designer is fun.
Me? I'm boring... I spend most of my day with Adobe CS3 products (I am currently playing with CS4 Beta and look forward to its release next year).
Macs are loaded with so much stuff, you could spend years exploring the tiny little nuances of its behavior - just today I discovered that it auto-detects headphones being plugged in [drag your volume control to mute position, and plug in some headphones as you watch the speaker icon in the menu bar].
If the above idea doesn't amuse you, browse around your drive and play with all the system software that isn't in your Applications folder.
CLIX is handy for Unix commands
Disk Inventory X or GrandPerspective is nice for seeing what your hard disk looks like.
SeisMac is cool but I live in California so it is pertinent.
xScope is excellent for design work.
Most of all, just shop around the internet and find stuff that YOU like!
Enjoy!