From Science News:
Its size is mind-boggling. With nearly 13 million digits, it makes the number of atoms in the known universe seem negligible, a mere 80 digits. And its form is tidy and lovely: 2n-1.But its true beauty is far grander: It is a prime number. Indeed, it is the largest prime number ever found.The Great Internet Mersenne Prime Search, or GIMPS, a computing project that uses volunteers’ computers to hunt for primes, found the prime and just confirmed the discovery
There's not a hope of printing it here: the resulting number would be 30 miles long! I figure you could stash it as 13MB or so of plaintext.
Largest known prime number found [Science News]
Distributed computing finds largest prime yet [ZDnet]