My team at work volunteered to help build a house for Habitat For Humanity this past Saturday. It was a great experience, especially because most of us were out of our element, doing the sort of things we would never have done on our own. I personally felt pretty useless, just standing around waiting for new things to hammer down. I definitely will volunteer again next year, or maybe (if I somehow stop being lazy) I'll volunteer on my own.
Link to the whole set at flickr: http://flickr.com/photos/aboatc/sets/72157604311733266/detail/