R you hip?
I've recently been playing around a little bit with R, which is an open source implementation of S-Plus. It's a powerful statistics package that seems to be very mature, reliable and flexible. The graphics it produces are very high quality and I would imagine it would be a great teaching package for statistics. I haven't tested its overall capacity, but it ran very quickly with the largest datasets that I had to throw at it.
Anyway, it is certainly worth taking a look at if you are looking for something that goes a little beyond Excel and don't want to shell out the big bucks for a commercial product.