It’s an 11,185,272-digit number which is available for download here. The larger one was discovered first on August 23rd by Edson Smith, who installed the prime-checking software on computers at UCLA. At 12,978,189 digits, it is now the largest known prime:
The two numbers were discovered by using a distributed computing project on the Internet called Great Internet Mersenne Prime Search (GIMPS).
A Mersenne number is a positive integer that is one less than a power of two:
Only 46 Mersenne numbers are known. Read more over at Wikipedia’s entry.