This family tree application looks really intriguing – you get to enter in your family tree and as your relatives sign up, they can add to it. Not only can you see all the relationships, you can contact them.
While I don’t usually worry too much about privacy on the web, this one actually seems a bit scary. All my relatives and their birth places on the web. And anyone can add the info. I could add it for my whole family without them even knowing. Note that you can keep the information private – it’s not necessarily published for the whole world.