Derive

I bought DERIVE and have thoroughly enjoyed it. I wish it were system compliant, but am nevertheless enormously pleased with it. It's been a real learning experience to work with this unusual program. With its built-in intelligence, it may or may not do what you want; it tries to guess and often guesses right, but not always. When it doesn't, it may still be possible to coax it. It's an experience, I tell you.

The 2 and 3-dimensional graphing is at least as much fun as any game, and can be quite instructive. I never realized just how interesting is the simple function f(x,y) = x^y until I tried graphing it.