My main project at the present is working with the good folk over at iThemes on BackupBuddy support and development on a part-time basis. Whilst I occasionally dabble with a few other things, some related to this and some not, this takes up most of my available time not given over to my other roles.
I have always had an interest in Processing, described as “…a flexible software sketchbook and a language for learning how to code within the context of the visual arts.” You can get a little more information on this here.
I love Mathematica and have used it on and off since its very early days – you can even get a free version that runs on the Raspberry Pi (although I haven’t tried it). Check out Wolfram Alpha as well, a great way to pass a few minutes and maybe learn a few things as well.
Along with Mathematica, one of my long term favourites is TeX originated by the great Don Knuth and its close companion LaTeX which I used back in the day for technical papers and the like. I even took a foray into Literate Programming which will be familiar to anyone who has read the original TeXbook and used it on some projects back in the day.