Great Internet Mersenne Prime Search
The Great Internet Mersenne Prime Search, or GIMPS, is a collaborative project of volunteers, who use Prime 95 and MPrime, special open source software that can be downloaded from the Internet for free, in order to search for Mersenne prime numbers. The project was founded and the prime testing software was written by George Woltman. Scott Kurowski wrote the PrimeNet server that supports the research to demonstrate Entropia distributed computing software, a company he founded in 1997.
This project has been rather successful: it has found a total of 7 Mersenne primes, each of which was the largest known prime at the time of discovery. The largest known prime as of December 2004, is 224,036,583 − 1. This prime was discovered on May 15, 2004 by Josh Findley. (There has been a preliminary report [1], [2] about the 42nd Mersenne prime being discovered, but no details have been published yet.) Refer to the article on Mersenne prime numbers for the complete list of GIMPS successes.
As of December 2004, GIMPS has a sustained throughput of approximately 16 TFLOPS, earning the GIMPS virtual computer a firm place among the most powerful supercomputers in the world.
Technically the GIMPS software is not open source, since it has a restriction which most open source/free software groups find unacceptable—users must abide by the prize distribution terms. This restriction will become meaningless when the EFF prizes are claimed.