Back in the day African Clawed Frogs were used as a pregnancy test for humans. More on that can be found at Wikipedia: http://en.wikipedia.org/wiki/African_clawed_frog

On another note... I worked in a herp lab for a while in college and took care of one of these guys - watching them franticly wave crickets into their mouths was always a funny thing to watch. As a pet I guess if you like other kinds of aquarium critters a Clawed Frog would be pretty interesting.