I don't think I've seen a group of fans as noticeably charitable as Firefly fans, aka Browncoats. Here in my home base of Austin, Texas they host charity screenings of Serenity on a semi-regular basis. Seems you really can't stop the signal.
Another group of Browncoats are putting together their own feature-length fan film, for sale on DVD with all proceeds going to directly to charity. The project, Browncoats: Redemption, features an all-new cast and crew aboard the Redemption on the eve of a second unified uprising by the Independents against the Alliance. The film is set to make its debut during Atlanta, Georgia's annual Dragoncon in September.
You can get involved with the film through Browncoats: Redemption's official website. I'm not usually one that pays any attention to fan films, but if there was ever a property that might get me interested, it would be Firefly. Its earthy, frontier setting means that a low-budget film maker could probably get away with something decent as long as they had a solid script and capable actors. I'm rooting for Browncoats: Redemption to pull it off.
(Thanks to Alison Nastasi for the link.)