Trying to build a computer algebra system. Basically like a small version of Wolfram Alpha, but customized and accessible by my own code so I can use it for whatever I want in the way I like to. It's starting to get cool enough to be usable for a bunch of things, but I'm still trying to figure out what I really want to use it for

It's challenging and interesting work, and I'd recommend building your own as a way to gain deeper understanding of math.