KDE is usually very nice, and (believe it or not) the Linux Kernel (Just make sure you know EXACTLY how to send patches and so on).

Although lately I haven't worked in anything, would like to hear about more great opensource projects.

Both Django and Rails are very inviting. There's also Plenrt of libraries for many languages that can provide a smaller, more specific place to start (it's often easier if you work with it regularly).

With many projects though, some conflate "my patch was rejected" for "they're jerks" without understanding the impact of their fix/suggestions. Feature bloat, backwards compat, API verbosity, etc all add up.

You could look here: https://openhatch.org/

Tor or i2p if you're into crypto and/or making the web safer.