
According to Aerohive, "This tool can give you an estimate of how many APs are needed in a deployment and where to place them. You can model the physical space that you're in, and see how the Wi-Fi network reacts when you drop in APs. As well as being useful for planning Aerohive deployments, because it estimates the coverage based on the radio type (e.g. 802.11n 3x3 MIMO), it also works for other vendors' products too. Finally, you can print a report of your planned deployment."
This planning tool is a subset of Aerohive's full product, the Online HiveManager.
It's super for proactive planning of your network, especially since it allows for customizing parameters such as the materials in the walls (brick, drywall, etc.). Of course, you can also import an existing floorplan.
Rather than either using trial and error or going through a repetitive cycle of placing APs and then measuring the signal strength, this is a great tool for knowing what to expect in advance as a part of an existing or new installation.
I've spent some time testing the tool, and it is indeed quite impressive.