Any good "working" screen savers available?

By which I mean, are there any that will carry out some useful work in the background when I leave my PC for the mail room or at lunch break? I did have the SETI saver installed at home but I was looking for something a bit more practical (that’s also not likely to be frowned upon in the workplace for chewing up the innards of our PCs)

I use the Cancer Research project here - other projects are also available.