World's largest prime number on three reams of paper
CNN reports that the new largest prime number was discovered on January 4, 2006. It is 9.1 million digits long. (thanks Tom for the link)
If you were to print this number out on 8.5x11" sheets of paper, one-sided with 0.5" margins at 11-point helvetica, it would take 1,556 sheets of paper. That's just over three reams of paper.