When I was a little kid, I had this relatively large, yellow calculator. I had no idea what it was called until I did a little Internet research yesterday. I matched photographs until one jogged my memory: the Little Professor Calculator (more info). Not only could it perform the basic arithmetic functions (I think), but it could test you with increasing levels of difficulty. It was a fun way to spend an afternoon before Nintendo, I suppose.
Well, if you get a kick out of calculators, you should love the mother of all financial calculator websites, hidden away at the Motley Fool. If the Motley Fool is the mother, then you could consider DinkyTown the great grand-daddy.