Building academic genealogy is fun and useful.
1. webbrain (I build it for my own knowledge)
2. http://academic.research.microsoft.com/
is currently in beta but has a cool visualization interface!
3. http://scholar.google.com/
has the largest database, but has no visualization interface.:(
4. Harvard Catalyst Profiles (for Harvard people only)
The interface is interesting but not friendly designed.
5. http://academictree.org/
basic graphic.
6. http://genealogy.math.ndsu.nodak.edu/
for Mathematics people. Text-based.
7. http://www.biomedexperts.com