This is an online project that has been around since 2003 and I'm surprised how often I have to explain what it is. It's been picked up by different kinds of media around the world, and there are almost 3 million users; hardly an underground group.
Personally I love to host and prefer it to actually surfing, as I don't really like to stay in other people's houses. Showing the city that I'm proud of is one of the most wonderful things to do and I try to make people feel at home, both in my house and in my town. Right now, unfortunately, I can't host all that much as my boyfriend's not a CouchSurfer, but I still show people around, once every so often.
I can't stress enough how much fun it is to make someone feel at home in a foreign country, to show them the non touristic route, to be their company away from home, to talk for hours and especially to see your city through brand new eyes. It always amazes me how easy it is to share the most private stories with strangers and how much you can learn from one another.