Volunteer Vacations....anybody done them?

So, a coworker just forwarded me an email from his older sister.  His sister just returned from Costa Rica (I want to go back!), but instead of partying and seeing the sights, she went down there to volunteer with Habitat for Humanity.  After her Habitat project was done, she did get to hang out with the monkeys in Manual Antonio, and watch Arenal erupt.....

Over the last year, I've thought about going on a volunteer vacation.  The basic premise is that instead of relaxing, you get to help others out....

While I can't swing another trip down to Costa Rica, I'm thinking about heading down to Louisiana to help out in the post-Katrina madness.  Do a little bit of good while visiting a place that I've never been...seems like a win-win to me.

Has anybody done this before?  Anything that I should know before I start booking flights???