I looked into this with the magic that is search, and it did not appear to have more than one post in any of the search results, so....
Basically, cranes that build things. Not cranes that move things, cranes that build things. The basic idea is, a crane will take the building item and build a building in the area it can reach (which extends both upwards and downwards). The crane would be powered by manpower or by mechanics, much like pumps.
Cranes could be built in a bunch of ways. First, the components, which would be wood, rope and mechanisms. Put some number of those together, and you can build a crane.
Second is to use crane parts, like the way siege weapons are built.
I originally came up with this specifically to make building stuff underwater a little easier. The idea is, you use the crane to make a piece of tunnel some depth into the water. You put in pumps and some mechanism for powering them as you go. When the tunnel is finished, you activate the pumps, pump the water out, and make it completely watertight, and you have a little tunnel underwater. You can then move on to the next section.
Of course, this would be limited by the crane's vertical range, so it's not as easy as it could be made.
But the crane would also be useful for building things that are really tall, or in a deep chasm...