GIMPS, located at http://www.mersenne.org/prime.htm, is the largest of several efforts to find prime numbers using idle time on ordinary home PCs.
GIMPS provides free software written in highly optimized assembly language. Using this software, individuals who own a Pentium-class PC with any major operating system can search for prime numbers with millions of digits. It is interesting to note that, despite the fact that each candidate number can take weeks or months to test, there is a larger chance of succeeding in a given time then in distributed.net or SETI@home
The clients are Prime95 and mprime, and are written by George Woltman.