Maybe it's just because I'm a nerd, but I think this is awesome: LibriVox is a website where volunteers read copyright-free books and other works then upload them. The result is books on mp3 for free for everybody, available online.
Lately I've become really fascinated by internet collaborative group projects like this one, and wikipedia, and even Yelp. (I'm not so sure what I think about citizen journalism yet.) I believe projects like these are going to significantly change the way we find information online. They already have for me. Yelp is already more useful as a city guide than CitySearch and DigitalCities, both of which have been around for years. I'll always click on a WikiPedia link before another one when it comes to learning more about a topic or definition. And the user reviews on sites like Amazon.com often tell you what you really want to know about a product.
What is interesting about these projects is that they are all about sharing knowledge, but the driving factor in all of them is showing off. For Yelp, it's showing off how cool you are. For Wikipedia, it's how smart you are or how much of an expert on a topic. With citizen journalism it's how fast you can gather and relay information. Peoples' contributions take hours and hours to prepare, yet all of these projects are things you can write while sitting on a computer at work.
Why is that interesting? Because of other potential applications that someone needs to be smart enough to think of: How can we use these extremely successful models of collaborative information sharing to effect real-world change in a more significant way than product reviews? What kind of problems could we solve this way?
All the information shared in the above examples is related to objects you've used or events witnessed or things you know about. Unfortunately, many of the world's problems are caused by ignorance (willing or not), so just putting out information on how to avoid homelessness probably isn't going to keep anybody off the streets. You can't make people read. The model would work better as a database of available services used by service providers (where are the shelters, free food, etc.), but those generally exist already. Volunteering is something you have to leave the house to do.
Wikipedia is essentially a humanitarian group project of organizing information. What can I collaborate on with people while sitting here at my computer that benefits people who don't have computers?
If you figure it out, let me know.