The technology of a rangefinder could be adapted to build something that I have always wanted: a device used to measure the distance between two remote things things (for instance the top of two flagpoles or the top of a telephone pole to the top of your house.) It would have a laser pointer that you would point to somewhere and press a button to indicate the beginning spot and then point somewhere else and press the button to indicate the end spot and then out comes the distance between the two points.
This of course, isn't the whole story, it's not a trivial problem to know what happens between the two button presses. Moving the rangefinder will mess up the calculation so this has to be compensated for (this is the reason survey folks have those little tripods and run around with mirrors on sticks.) One way to solve this problem is to put accelerometers in the device so that you can keep track of how the device is moved. Alternatively it could have two rangefinders connected by a protractor type thing to measure both at the same time.
An add-on would be to be able to tell the area of some surface outlined by some number of points.
It's important to note that this is different from current distance measures in that you don't want the distance from the object to you, you want the distance from one distant object to the other distant object. If you do the math, you can quickly discover that trying to work out all the angles and distances in 3d is quite complicated and I have never known anyone to sit down and do the math when it's probably less work to try to work it out another way. The device isn't irreplacable (any more than a rangefinder is, just get out the tape measure) but it would make things MUCH easier in many cases.
The Custodian has pointed me to a laser rangefinder/Digital Compass/GPS combination developed for the 21st century land warrior project that will allow soldiers to point at things and be able to transmit the coordinates to the folks with the big guns so that they can destroy them. I think what I want would be a little more presise in terms of the distance between two objects, with differential GPS, this accuracy can be increased but at the cost of extra money. Now if I could just figure out where to buy one of these things.