There are many situations where I don’t think technology brings much “value-added” to a lesson. One of them is a common learning activity used with Beginning English Language Learners — having them create family trees.
It’s a great way to develop basic vocabulary, and you can expand it to include other descriptions of family members (what kind of work are they doing or did they do?).
Student can create beautiful posters for the classroom.
If, however, I’m wrong, and somebody can think of a benefit to students doing it online, Dynastree is probably the most accessible (and free) application out there.
It does happen to have a free and easy mapping tool where you can see where other people who share your last name live in the United States, and that could be a useful application to learn geography. However, that tool seems to be down at the moment I’m writing this post.
I’ve posted in the past about a similar, and I think better, site that maps your namesakes around the world. That one is called Public Profiler/World Names. You might want to read my previous post on it.