As a hobby, I like running large mathematical computations. My favorites tend to be the mathematical computations that may appear to provide little practical value, but there is a lot of value gained in learning about methods to run and optimize large computations.
y-cruncher is a proprietary software by Alexander Yee which supports computation of various irrational constants to high precision, such as pi. Click here for the page about my computations with y-cruncher.