For those of you that don't know what a Mersenne Prime is (most people I suspect), it's a prime number of the form 2^n -1 where n is a natural number. The largest known prime number, 2^32,582,657 -1 is a Mersenne prime. Many, no doubt fascinating individuals (like me for example) like the idea of finding the first prime number that is > 10 million digits long because 1) there's a $100,000 reward and 2) because it would be cool (sort of). Check out GIMPS and Wikipedia for more info.
I searched around for ages for a version of mprime (highly optimized software used to search for Mersenne Primes) compiled for Intel Macs and didn't have much luck; for some reason it doesn't seem to be linked from the GIMPS website. Eventually I found a random link to this page which contains all the software downloads. So if you want to contribute to the worldwide distributed search and you use a Mac, please download it. And you never know, you might get lucky!
For those of you that couldn't care less about Mersenne primes (most people I suspect), but want to do some stress testing/benchmarking on their Mac, mprime is a very useful tool. Overclockers of the Windows variety will be familiar with the Prime95 app, a Windows GUI version of mprime.