Gene Geni
This is quite clever - although it's a pity it uses Flash. You start to build your family tree on-screen, adding emails to the names where available. These are then sent info about the site, and obviously encouraged to add their own local knowledge of the tree. So the system is viral, and is based on two networks: that of family connections, and that of the Internet.
It's easy to foresee the day when we know all our public genealogical connections in this way - a stage before our genomes are used to show all the private ones, too.... (Via TechCrunch.)