The Griffin Technology PowerMate has been around for a while. This thing is entirely frivolous. There are a few use cases where it is amazingly useful, mainly video editing. But when you touch it and spin it the feel is just amazing. It has heft, and a superb spin that just makes your spine tingle. You can click it by pressing it down. Want to select a range of video in Final Cut, simple spin, press and hold, spin, release. Done. So much more intuitive, and just fun, than dragging a mouse around.
My Mac Pro now has a knob. Yeah!