Earlier, sourceforge had up-to-date listings of volunteer requirements. Now, very few projects are asking for volunteers. Is there any other forum where requirements are posted, and people interested in particular languages/technologies sift through the listings and apply for volunteering ?
Sourceforge still has a section called Help Wanted System. Currently there are over 100 "job listings" so you should be able to find a project that you find interesting.
Mediabird also seems to be looking for people with all kind of programming skills.
As far as can tell, they're all looking for help. Pick one you're passionate about, contribute some patches, and if they're accepted you've found a home.
Are you looking for a specific language or technology?
Find a project you are interested in and see what you can do for them. Most projects have forums and mailinglists. It never hurts to ask. Aside from that @Espo already pointed out that SF still offers that section.
Jon Galloway is right, you need to pick something you're passionate about.
Personally I think it's important you choose something that you use. There's no better way to get motivated to fix a bug than if you hit that bug every day.
You can also try hanging out on the mailing list to get an idea of how the development process works and if you'll gel with the other developers. You're not going to want to spend your time fixing bugs with people you don't like.
I have had no problem finding projects to get involved with through the sourceforge listings. All you have to do is find one that interests you and then contact one of the project leaders. They are usually pretty quick about getting back to you.
It's been suggested already but browsing SourceForge is a good way to find OpenSource projects looking for help. I think Google Code may also be good, but I'm not sure, as I've not seen much projects hosted there (Doesn't mean they are not in there, I just haven't seen them)