I found this one on the Chrome Experiments page. Play a trick on Grandma and tell her that she broke the internet! Let her tap the top of the screen before it happens. The "gravity" doesn't set in until your mouse moves over the screen. (Make sure you search for something and watch the results fall down as well.)
This is another fun one. Watch all of the other links swirl around the Google logo as you move your mouse.
After the page loads, click anywhere on the page and after 5 seconds the "oo" in Google disappears. Click again and they reappear. If you rub your fingers on the screen while the "oo" is disappearing, it adds a nice little affect.