Not actually the first HHH, but the first one that will be easy to find.
hairdryers have a multitude of uses: Dry your hair, dry someone else's hair, act as a stand-in ray-gun, useful as a suicide instrument when combine with a bath tub full of water, the list goes on.
You can now officially add useful painting and modeling tool to that list.
Safety first: Hair dryers produce heat. Be careful with you hands and fingers when handling anything you are aiming the hairdryer at. Be careful you don't overheat plastic models as well.
As a general rule, use the hairdryer on lower settings and try to use sweeping motions to heat the area your working on evenly. You don't want to burn your fingers or melt plastic.
Here are three ways I've gotten the most millage out of my hairdryer:
1) Use it to dry paint quickly. When you work with drying retarders as much as I do, it can get frustrating to have to wait longer for paint to dry. Use your hairdryer to speed this process up. Be sure to use the 'low' setting and wave the hair dryer over the area (avoid direct )for best results - i.e. to avoid blowing the paint around and to avoid any kind of damage to the paint or model itself.
Bonus: I even her rumors that this will increase the toughness of the paint to some degree.
Warning: I wouldn't use this on any paint that has been applied with an airbrush. The layers are super thin and when I did this, my paint crinkled.
2) Use it to dry other things quickly. I've gotten the most use out of my hairdryer when using weathering powders. One common technique is to brush on the powders and then soak them with some form of thinner (I use white spirits). This can take a while to dry on its own. The good ol' hair dryer speeds this up and even makes the powders more resistant to spray varnish.
You can be a little more careless with this use. I've had good experiences with full blast really baking the powders on. But be careful not to melt any plastic you may be working on.
3) Use it to soften up resin models. Resin models have a nasty habit of warping during the casting process. Fortunately, resin is pretty receptive to heat when it's cured. You can use the hairdryer to heat up areas you need to bend into shape pretty easily. I found that using a hairdryer is faster and easier than the hot-water method. Standard safety warnings apply.
Lastly, you only need to spend about 10 bucks on a hair dryer to get a tool that will meet your needs.
Unless you have some serious work to do on your hair-don't.