At least for now, the least needed tool I have seen is for sale at Home Depot.
The Ryobi Auto Hammer!
It is an $89 tool that you push on a nail and it hits the nail over and over.
http://www.homedepot.com/webapp/wcs/...r%20Img%20-D25
Let's see if you can top that for the "Least Needed Tool"!