Great Internet Mersenne Prime Search (GIMPS):Large distributed project to primality test all Mersenne numbers with free software by George Woltman. Increased largest known prime many times.
Ars Technica Team Prime Rib: A distributed computing team dedicated to finding Mersenne prime numbers.
Internet PrimeNet Server: PrimeNet is a distributed computing project for the Great Internet Mersenne Prime Search, co-ordinating the assignment of work and collection of results.
Marin Mersenne: Biography of the Minim friar and his contributions to number theory. Includes references, pictures, and cross-references to topics.
Mersenne Prime Digits and Names: Landon Curt Noll lists the decimal, English, and American expansions of all the known Mersenne primes.
Mersenne Prime Mailing List: Unmoderated list for communication between those interested in all aspects of Mersenne Primes. Sign-up and access to archives.
Mersenne Wiki: Information about the Great Internet Mersenne Prime Search, other distributed prime projects, and the related mathematics.
mersenneforum.org: A forum about prime numbers and computerized searches for more of them. Includes discussions of software and hardware issues encountered by searchers.
The Prime Pages: Mersenne Primes: History, theorems, tables, conjectures, heuristics.
Will Edgington's Mersenne Page: Files with Mersenne factors. Includes proofs of theorems related to the search for Mersenne primes and factors.