Michael Shafer, a CHEE grad student at Michigan State, has identified the world largest prime number to date -- 6,320,430 digits. Michael is a participant in Gimps (Great Internet Mersenne Prime Search). Marsenne primes are of the form (2^P)-1.
The number took a total of 25,000 computer years to discover (Cumulative time on the 60,000 participants computers). Once discovered it took Michael's 2 GHz Pentium 4 19 days to verify that the number is prime.
The number (= 2^220996011-1) is so long it would take almost 5 weeks to write out by hand. Printed in 12 point font, it would be over 16 miles long (22 miles with commas)! An over-sized poster in 1 pt font is available for serious math nuts. Source