SETI @ home
As over 4.5 million other people, I'm running a SETI@home-client on my computer.
I used to run the client 24/7 on my desktop-PC at the MPI Dortmund as a background-job under Linux and accumulated a total of 2.956 years of computing time (on the old client).
Absolutely not impressive, if you compare this to other peoples numbers, but...
Most incredibly, I was one of approximately 7000 people who processed a data package with some sort of recurring signal, which has been selected for re-observation by the project-leaders.
Addendum: Because no aliens tried to phone me in the meantime, I assume, that my candidate signal was sorted out on reobservation.