One of the major HVAC instrument companies is Fieldpiece. See: http://www.fieldpiece.com/
The anemometer accessory is here:http://www.fieldpiece.com/accessory-heads/aav3
I've got the complete instrument kit for HVAC and gas furnaces. The anemometer is used to balance the air flow in a heating or cooling duct system. Basically you measure the air flow at each register and balance the system by opening or closing adjusting vanes as needed.
Some high efficiency heat pumps use a control system that sequences heat to each room in turn. It's done by opening and closing the motorized duct dampers. The anemometer instrument is necessary to adjust the system.
As "coaledsweat" points out a manometer and a pitot tube can also make the measurements. It's more difficult to to use than the anemometer. If all you want to know is relative air flow just tape equal lengths of toilet paper to the various registers and just compare the deflections. Rule of thumb for residential A/C is 400 CFM per ton, 450 CFM for a heat pump. There are ways to calculate your existing heat flow by putting a heat pump system in the electric heat mode only. Then by measuring the electric power input and the temperature rise over time you can calculate air flow.