I am a former kernel developer of Mathematica (2001-2008). Most notably, I designed, programmed, and documented the new NIntegrate (since version 6). I also worked on other projects mostly related to numerical computations. (Like linear algebra, numerical summation, clustering, elementary statistical functions, wavelets, matrix functions.)
At some point I was also involved in WolframAlpha. I programmed the initial version of the chemical equation balancer, and later supported and extended it. I also programmed the initial version of the music scale related functionalities and displays.
Currently I use Mathematica mostly to develop machine learning algorithms, search engines, recommenders, conversational engines, for data analysis, and, of course, for general research and development.