Recently Kaspersky company found a new virus that encrypted your hard disk files using a RSA key of lengh 1024 bits, that is 1024*log2=309 digits. kaspersky asked for a collective effort to crack it (for the purpose of recovering the encrypted data).
A RSA key is a number (in this case of 308 or 309 decimal digits) which is simply the product of 2 primes of about the same number of digits (154 each).
You have to factorize (find the 2 primes) to crack RSA encryption, but the estimated time to do that is of 1 million modern PC´s working together 24 hours of a day for 15 years. That´s too much time for a distributed and very interesting project.
See:
http://www.schneier.com/blog/archives/2008/06/kaspersky_labs.html