From www.mersenne.org…

*On November 17, 2003 Michael Shafer’s computer found the 40th known Mersenne prime, 2 ^{20,996,011}-1! This number “weighs in” at a whopping 6,320,430 decimal digits! This is also the largest known prime number, surpassing GIMPS’ last discovery by over 2 million digits. Here’s the number in case you are interested.*