The existing "Bugs Ahoy" site (created by the same person) has lots of choices: http://www.joshmatthews.net/bugsahoy/

...but it gets you right to bugs, which may be a harder place to start for some contributors.

I would like to toss in that I am a beginning programmer: Not nearly good enough to fix the bugs, but I can read code and help comment it, change it to make it stylistically compliant, etc.