I've written a ton of code for my employers, but I'd like to contribute to some open source projects like Angular or JQuery UI. I'm not sure where to start.
This is definitely one of the toughest parts of leisure programming. The goal is to always have something interesting and fulfilling for yourself to hack on in your free time. Personally, I find these things by either coming up with a feasible idea of a new project or finding an existing accessible (and smallish) project through reddit or HN. I've also found projects as dependencies for other projects that were in need of maintenance. Do what works for you, and try to keep your ear to the ground. And the last bit of advice is to write everything down, even if you think it's a bad idea, because you will NOT remember everything. Good luck!
