Some time ago I read that Google had acquired SketchUp (a 3D drawing application with an impressive (and patented I think) drawing system that allows you to build 3D objects with strokes of the tablet/pen or mouse). I believe this was around before the Tablet PC. My theory is that if it had been launched afterwards and with some fanfare, it could have been that killer-app the tablet is seemingly still looking for.

Well anyway, now it's available (in a simplified form) for free.