[Suicide Junkie]

What happens is that you download a small program, and it will run in the background while your computer is on.
It does a pretty good job of only using your idle processor time, so it will slow down or stop its work while you run some other application.

It will first download a chunk of raw data, and then grind away looking for patterns.
After roughly 10,000 Mhz-hours, it will finish, send the results back, and get another unit to work on.

If you're on a high-speed connection, you can set it to autmatically connect. If you're on dialup, it is probably best to have it wait. In that case, you will see the icon blink, and when you're connected, you can click it and press "connect now..." to send the results.

The total work done is tracked by email address, so if you have multiple machines, you can enter the smae address into the settings, and they will all count towards your ranking.