Geni

Geni.com is a new ajax-based site that allows you to easily create an online faily tree. As you add your family to your tree you can invite them to contribute. (It's slightly social from that point of view but I'd wouldn't call it a "social web" application due to a lack of many other "social" features.) I can say it's very easy to use but I'm waiting to hear what my father thinks of it. (He's the family genealogist.)