The largest prime number known to date - the 48th Mersenne prime - was found on January 25 at the GIMPS project, as has been the case for all record primes since 1997.
A number, N, that can be represented as 2n - 1 is called a Mersenne number after Marin Mersenne, a French philosopher who first studied such numbers over three centuries ago. When both N and n are prime numbers, however, N is called a Mersenne prime, a number very rare and difficult to find on the number line - even by supercomputers clocking hundreds of trillions of calculations per second.
Mersenne primes have limited practical applications, but the few themselves are very interesting. For instance, if you've played the game Tower of Hanoi, you'll know that solving an x-disc tower requires as many moves as the closest Mersenne prime to the xth power of 2.
On January 25, 2013, the largest prime number yet - the 48th Mersenne prime - was discovered: It's value is 257,885,161 - 1 = 581,887,266...724,285,951 - the entire number in text format is available for the viewing here. Beware: The number contains 17,425,170 digits and is stored as a 22 MB text-file.
The discovery, like that of all the major primes since 1997, was made at the Great Internet Mersenne Prime Search (GIMPS), a distributed computing network over the WWW. Specifically, it was made on the computer of Curtis Cooper, a volunteer and mathematician. The network itself involves 360,000 CPUs peaking at 150 trillion calculations per second. In comparison, the Jaguar supercomputer - one of the world's fastest - clocks 1,750 trillion calculations per second at its peak.
Interestingly, this is the third Mersenne prime involving Cooper's system, although the first two, M43 and M44, were discovered on Cooper's and Steven Boone's 700- and 850-PC cluster, respectively.
The validity of M48 as a Mersenne prime was established by testing on four different machines, one of them running the MLucas software, two the CUDALucas software (with one of them on an NVidia GPU), and one using the GIMPS itself on an Intel i7 CPU.
For his discovery, Cooper will receive a cash-award of $3,000 from the GIMPS' board of directors. The GIMPS itself, on the other hand, will now go after the $150,000 up for grabs from the Electronic Frontier Foundation (EFF) for finding a prime (not necessarily a Mersenne prime) with 100,000,000 digits (i.e., "100 million digits" - whichever sounds more ominous to you).