SETI@home was the first project I joined that used the spare time of my computer. I have always been interested in Astronomy and the chance to take part in a research project to try and detect signals from alien worlds interested me.
The SETI project is privately funded to buy time on the Arecibo radio astronomy dish. Chunks of the data collected are then sent out to the home computers of thousands of people where it is analysed and searched for unusual signals that may be alien. If something is ever discovered the owner of the computer will be partly credited with being the first person to find evidence of alien life.
When the BOINC programme was launched I converted to using it and continued to search for these signals.