[mmaunder]

I'm not sure what languages you know, but WordPress is a great one if you can program in PHP. The code is easy to read and there's plenty of room for contributions and improvement in WordPress core. Also the community is really friendly and welcoming.

[csixty4]

There's even a list of "Good First Bugs for New Contributors": https://core.trac.wordpress.org/tickets/good-first-bugs

[jacorreia]

Yeah, I've heard great things about the Wordpress community! Some of the others that I've found have a great support base are Open Streetmap, Open Science, and Ubuntu.