Garmin R10- Launch monitor that gives the 20 most important parameters for just $600, not thousands. Seems to be accurate, a great help to giving a quality tech-driven lesson.
Orange whip- an old favorite that seems to definitely help beginners and high handicappers; jury is still out on whether it is much of a help to skilled players.
Ordinary 21/8 in. wooden tee- feedback from just a common tee can be very helpful in determining path and angle of attack.