Great Internet Mersenne Prime Search
The Great Internet Mersenne Prime Search, also known as GIMPS, is a prime example of distributed computing project at work and no pun intended. It is a collaborative project of volunteers, who use Prime95 and MPrime, software that can be downloaded from the Internet, in order to search for Mersenne prime numbers.
Mersenne primes are named after Marin Mersenne, a French monk and mathematician, who was born in 1588. Mersenne investigated a particular type of prime number: 2p - 1, in which p is an ordinary prime number.
Mersenne primes are much rarer than ordinary primes, of which there are an infinite number. The GIMPS effort, exhaustively searching for possible candidates since 1996, has been responsible for discovering the largest Mersenne Primes to date. See the list of Mersenne Primes for more details.
This project has been rather successful: it has found 16 Mersenne primes, each of which was the largest known prime at the time of discovery. The largest currently known prime is 277,232,917 - 1 (M50). Refer to the article on Mersenne primes for the complete list of GIMPS successes.
The project was founded by George Woltman, who also wrote the prime testing software Prime95. The GIMPS project was formed in January 1996. Scott Kurowski wrote the PrimeNet server software that supports the research to demonstrate Entropia distributed computing software, a company he founded in 1997.
The Electronic Frontier Foundation offered $100,000 to the first person(s) who discovered a ten million digit prime. GIMPS claimed this award. The owner of the discovering computer received $50,000 from the GIMPS foundation.
Although the GIMPS software has its source code available, technically it 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.
Most GIMPS members join the search for the thrill of possibly discovering a record-setting, rare, and historic, new Mersenne prime. All you have to do to be part of it, is to contribute spare or idle CPU cycles. Pretty cool.
If you want to know more, there is a GIMPS FAQ available in this wiki.