Ask HN: Need help finding open source projects to contribute.

I recommend contributing to projects you already use. That way you'll have a better idea of what specific features or issues you should work on. Also, you'll be more likely to succeed, since you'll be motivated out of self-interest as well as general goodwill toward the community.

Standalone applications that you yourself use will be more rewarding.

Beyond that it is hard to say where to start; are you a C coder? PHP? Perl? Javascript? Ruby? If you gave even a hint of your level of experience and your preferred language/environment then people could be more useful.