OK, I know that for landscaping purposes, developers / construction workers line up two of those little telescope things to look at the other one and take measurements or something, but what exactly are they doing? My best guess is for water runoff…
They are used for surveying and to develop a topography map. The thing is set on a tripod with known height (usually 5 ft), leveled and then looked through to someone holding a vertical “yardstick”. (at least that’s the way we used to do it). There are other ways to do this of course, such as measuring the distance between the two and the angle of the sight, but the results are the same. From numerous readings the elevations are determined. The topography map is used for absolutely everything in design and construction, not just water runoff.
-Beeblebrox
“I teleported home one night
With Ron and Sid and Meg.
Ron stole Meggie’s heart away
And I got Sidney’s leg.”
And, to supplement Beeblebrox’s post, the device is called a surveyor’s transit or transit theodolite.
They “telescope thing” is called a transit and the “yardstick” is called a stadia rod.
I believe they are used to measure distance using parallax. The rod stays out while the transit is used to make various angular measurements. Using trig, the distance from the transit to the rod can be calculated. Do a web search on ‘parallax’ for detail, or try my own page at http://www.badastronomy.com/bitesize/parallax.html for how this is used in astronomy.
Thanks everybody! Now I can tell Mommy what I learned today. 
Former surveyor checking in…
You could be looking at one of several instruments, as mentioned above either a level (most likely on a construction site), or a theodolite. Most theodolites these days also have an attached electronic distance measuring device attached above the scope, which can help you determine what instrument they are using. If they are sighting another tripod, it is most likely a theodolite EDM combination and they are sighting a mirror or set of mirrors that reflect the measuring beam back to the EDM.
Shooting stadia is an old, seldom used method anymore for determining rough distances. A stadia rod is a different critter than a leveling rod. Stadia is marked off at one foot intervals and is usually orange and white. A leveling rod is deliniated into feet and inches or in feet and tenths in most of the U.S. and in meters and centimeters in most intelligent countries.
Stadia in surveying is not measured using paralax, it is measured with stadia hairs or the two extra horizontal hairs in a transit or theodolite.
…and if you really want to fit in on the job site that rod is called a story pole. Just like when a level is held vertically is called a plumb stick.
(Just trying to get my post count up so that people won’t giggle…)