Caffeine keeps a computer from locking up or falling asleep by regularly simulating keystrokes. Like a strong cup of coffee, the program prevents your screensaver from kicking in without your having to disable the screensaver itself. It's handy for times when you don't want a screensaver but also don't want to change your desktop settings.
We enjoyed this program's interface, because it didn't try to overextend itself. Caffeine basically has only an on/off switch. It's pretty much impossible to be confused with such a simple layout. The program's description claims it simulated a keystroke once a minute in order to keep our computer from going to sleep. We had no way to prove or disprove whether the keystroke happened, but the program lived up to its billing: we left the computer idle for several minutes, which normally would have resulted in the screensaver kicking in, and nothing happened. Since that was what was supposed to happen (or not happen) it is obviously an effective way to keep your screen open. Caffeine is so simple and basic that it offers no features, special or otherwise. While this doesn't hurt its overall excellence, a timer or scheduler might be a smart inclusion. Regardless, this was a great way to keep a computer from lapsing into sleep.
Caffeine is freeware. It comes as a compressed file.