I have been donating cycles to the Great Internet Mersenne Prime Search (GIMPS). http://www.mersenne.org I started this about 8 years ago (IIRC), with a client that had manual communication (emailling handwritten results back and fourth). The computer running the numbers didn't have a direct internet connection at the time! It took me about a year to trial factor a number by about 60 bits. Now, I replaced that 486/DX2 66 with a Athalon 1600+ which gets that done in under 2 weeks. I have donated over 22 pentium90 years of cpu time, putting me in the top 5% of the 20,000 current users. They are currently running at over 1.5 terraflops.
They are looking for primes that follow the equation (2**n)1. (more info at http://www.utm.edu/research/primes/mersenne/index.html) As a previous poster has mentioned, only 39 of them have been found, most in the recent past. As you could imagine, these numbers get big really quick. Hundreds of thousands of digits big. The best part is, if your computer helps find a 1 million digit prime, you will get a significant portion of a $100,000 USD reward.
The client software is open source, and VERY well written by George Woltman. It is probably one of the most optimized distributed clients out there. They have a mailing list with some rather indepth discussions about number theory, algorithms, and processors in general. Its an organization that you can actually participate in, rather than just blindly assist. You also get to see the results of your work, unlike some of the distributed clients.
So, if you are looking for a good interactive community to join, with well written software (why waste precious cycles!), and results you can see, join GIMPS!!
Speeding never killed anyone. Stopping did.

