Geni
I found a sweet web app Geni. It has one of the slickest interfaces I have ever seen. It is a social networking site based on family trees. You build out your family tree and invite the members of your family to join.
Among other things, it is a really cool way to keep track of family birthdays and compile your genealogy.
I am also excited about its ability to be a “gateway drug” into social networking sites for people who would otherwise not see the appeal