In my opinion Prime95 is a very interesting project + they are awarding $100,000 to the winner!
You could discover one of the most coveted finds in all of Mathematics - a new Mersenne prime number. We've found four already. Join in on this fun, yet serious research project. All you need is a personal computer, patience, and a lot of luck.
In addition to the joy of making a mathematical discovery, you might win some cash. The Electronic Frontier Foundation is offering a $100,000 award to the first person or group to discover a ten million digit prime number! See how GIMPS will distribute this award if we are lucky enough to find a ten million digit prime.