I am currently using Mathematica personally for two activities (I also use it professionally as a mechanical engineer). But these are my personal interests:
Image software - especially imaging for pattern recognition in digital photos, and artistic effects on drawings and animations.
I have been writing a general simulation code that looks at future trends in the world e.g. population growth, available food supplies & resources, build-up of pollution, birth rates and mortality rates, wealth distributions etc. The code is intended to be a successor to the old study "Limits To Growth" that was done in the 1970's. The goals are the same - to see if current world trends are really sustainable, and if they are unsustainable then where/when do things break down?? The work is being done slowly & patiently - currently is not funded by anyone.