Check out OpenHatch http://openhatch.org/. Connecting people with time to projects with need is a big part of what they do.

Every Open Source project needs people. Find one that is doing things you are interested in and take a look around for things to do. Be sure you like the vibe of the project as well, do the people seem like folks you'd want to hang out with in your free time? It's not supposed to be a boring or tedious job, you may already have one of those so it should be fun.

I'll put in a plug for jQuery, see http://contribute.jquery.org for some guidance. There are plenty of things to do in every project besides hardcore coding as well, such as documentation or build systems.

In the continuity of OP's post, I'd like to introduce sup [0], a MUA project written in ruby a long time ago by William Morgan to put gmail functionalities in your console. It has seen quite a success (even creating a fork-for-speed, notmuch [1]). Unfortunately William has left the boat, and sup is now maintained by other people. It's still hackish and has some bugs, but it does the work fairly well. Help is highly welcome !

[0] supmua.org

[1] http://notmuchmail.org/

Check out quill.org. We're an open source tool for learning English grammar. We've got an active community of developers at https://github.com/empirical-org/quill, and we currently have about 500 students using our site per day. There are a variety of front end and back end projects. Feel free to reach me at peter (at) quill (dot) org.

I'm sure there are plenty! You could start by picking a project on GitHub that seems interesting. Have a look at the open issues to see if there's anything you could fix. Most repository owners will be very grateful for pull requests, and are usually happy to provide feedback on your code.