I've installed the breveCreatures screensaver at work, which has been entertaining my co-workers whenever I'm not at my desk.
It sets up a simulation that evolves a walking creature based on a simple genetic algorithm. The state of the simulation is saved each time the screensaver is interrupted, the state is saved, and will pick up again the next time it starts. Very entertaining eye-candy.
I've also got so used to certain aspects of Firefox extension functionality, I've installed SafariStand to enable my typical browser session state to be saved and loaded with ease. Not quite as nice as having it saved when you quit, but it'll do.