I'm not actively involved in anything in a volunteer capacity at the moment, but for pretty much the entire decade of the 90's I was a volunteer firefighter.
Now, firefighting is too big a time sink for me, especially in terms of being as active as I was back then. But if I could free up some time to do something volunteer related, my first choice would be to volunteer as a tutor for the local Literacy Council.
I've also toyed with trying to squeeze in some time to teach some free programming classes at the local hackerspace, including maybe a kid centric thing using Scratch or something of that ilk. But right now I work a full-time job AND am working on a startup, so I've kinda put that stuff on the backburner for a while.