I've been using the SETI@home software for over a year now. It's a little programme that you download, which then uses your computer's "down time" to analyse data collected by various radio telescopes. Having completed each "unit", it uploads the results and downloads the next unit.
I works like a screensaver, so it doesn't slow your computer at all. The practical upshot is that, with vast numbers of computers processing units daily, the SETI@home project is like one vast supercomputer. In fact, SETI@home processed its 2 billionth unit early in september.
It's a great project, you lose nothing (because it only runs when you're not using your computer) and you're contributing to real science.