Rangefinders are monoculars that direct class 1 laser beams to the target. The time delay in the laser beam, as it travels to and back from the target, is measured and used to calculate distance.
As an example, the Leica LRF 1200 Rangemaster has a range of 1100 meters. It can estimate the target distance with an accuracy of approximately 1 meter when the range is less than 400 m, 2 meters when the range is less than 800 m, and about 4 meters at greater distances. It only weighs approximately 320 gm (11.25 oz) with batteries installed. The magnification of this monocular is 7×. It has nice features like water resistance and the ability to display the distances for multiple objects in the field of view. It can be mounted on a tripod. The cost is approximately $470.
Bushnell manufactures reliable but less expensive models. While Bushnell's models are more directed towards the golfing or bowhunting markets, some have nice features (e.g. the Yardage Pro Legend WP is waterproof).
The most advanced rangefinders also incorporate internal compasses. These allow the distance data to be converted into a relative vector coordinate. If the precise location of the observer is known (via a GPS unit), the precise geographic location of the object observed in the rangefinder may be determined. For example, the Leica Geovid 7×42 BDA has 1-meter accuracy and a range of 1000 meters. The Geovid can function with azimuth inclinations of up to 35°. This 7× model weighs 1490 gm (52.4 oz), which is nearly five times the mass of the Leica rangefinder described above. Using this will build your biceps and triceps! A 2002 web search for the Geovid BDA on the retail market revealed prices between $2400-$3600 (list price: $3995).
Unfortunately, Leica discontinued the BDA, and replaced it with the BD model that does not have the internal compass. As far as I can tell, the BD model offers little you cannot get from a conventional rangefinder. In a May 2002 web search, street prices for the Geovid BD model ranged from $2150-$2600 (list price: $2595).