Iterated functions are what makes fractals such as the Julia Set and the Mandelbrot Set, so if you like those, you'll like Iterate!, too. In fact, Iterate! has some powerful tools for studying Julia Sets and how they relate to the Mandelbrot Set. But best of all, Iterate! is free! (OK, OK, it's not really free, it's shareware. But just go ahead and use it without telling me--that's what everybody else does!)
3DFract draws 3-D fractals and lets you rotate them, zoom in on them, and otherwise manipulate them. This is the New and Improved Version 2, which has 10 new fractal types, the ability to output the fractal in POV format, and some other great new features. File size is around 100K.
NoteTest is a set of computerized flashcards for the beginning pianist. The computer displays the note, the student must respond by playing it on a MIDI keyboard. Requires MIDI card and keyboard.
EarTest, an ear training program. EarTest plays a note, you must respond by playing the same note on a MIDI keyboard. It has several levels, from very easy to incredibly difficult. Requires MIDI card & keyboard.