Details from the Great Internet Mersenne Prime Search:
On January 25th, prolific GIMPS contributor Dr. Curtis Cooper discovered the 48th known Mersenne prime, 257,885,161-1, a 17,425,170 digit number. This find shatters the previous record prime number of 12,978,189 digits, also a GIMPS prime, discovered over 4 years ago.
More information can be found in this article from New Scientist. Those interested in the workings of the GIMPS algorithm should visit the GIMPS Mathematics and Research Strategy page.