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This is by far the energy and building science professional's most useful tool.  It's called a blower door, and it consists of a calibrated fan and a digital pressure and flow gage.  The flow portion of the meter measures the volume of air flow through the buiding in cubic feet per minute.  This can be very useful in determining how much airsealing work needs to be done.  The higher the reading in cubic feet per minute, the more airsealing needs to be done.  The blower door is also used to perform pressure diagnostics.  This is simply measuring the difference in pressure with the fan running between heated and cooled areas and unconditioned areas such as attics and crawlspaces.  This demonstrates how connected unconditioned and conditioned areas are with one another.  Ideally, it is desired that there be no communication between them.

Pressure readings can also be taken to test how much heated and cooled air is being lost due to leakage in the duct system.  A hood (pictured above) called a pressure pan can be used in conjunction with the pressure and flow gage to test each supply and return register to pinpoint areas that need sealing and repair.