Setiathome

From the website:

SETI@home is a scientific experiment that uses Internet-connected computers in the Search for Extra Terrestrial Intelligence (SETI). You can participate by running a free program that downloads and analyzes radio telescope data. A command-line only version is available for Linux and other Unix based operating systems.

A similar, but more flexible, program is called BOINC. This allows you to analyze any data someone wishes to let you. There are a great number of projects out there requesting help, and you can select to run as many as you want (although only one at a time).