Distributed.net
distributed.net is a world-wide distributed computing effort that is attempting to solve large scale problems using otherwise idle CPU time. It is similar to the SETI@home project in that sense.It started out as an effort to break RC5-56, a 56-bit encryption algorithm that had a $10,000 USD prize available to anyone who could find the key. That challenge was solved on October 19 1997 after 250 days. The next project was the RC5-64 challenge which took nearly five years to complete before the correct key was found on July 14 2002.
Currently, distributed.net is working on RC5-72 (breaking RC5 with a 72-bit key), and the OGR-25 project, which is searching for 25 point Optimal Golomb rulers.