For me it was helping the homeless. There are many ways to help homeless people, but I think the best way is showing them that someone still cares just if even to only help serve a meal. The St. Vincent de Paul Society has many things here you can do. I suggest serving meals where you get a chance to interact with them. The meal is great way to help at a basic level but where you can do these people the most good is to just smile and ask how their day is going... Show them kindness. Give them hope.
That is the way I feel I can best help the homeless at this moment.
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.