And watch out for those sling psychrometers! They hurt when you get hit by one, and they're horribly inaccurate when not used properly.
If you're a design engineer, a good AutoCAD program and some reference books will do. If you're an energy-saving-analysis engineer, you'll need to analyze lots of things in the field, but it's better to acquire the test tools as you find a real need for them.
Good on ya,
Goober Dave