If you are making sugar based candies, the one item you simply cannot do without is a good candy thermometer. Some thermometers meant for frying oil may have a range that is too high for candy—for toffees and caramels, we'll be in the 100-300 degrees Farenheit range. Fortunately, candy thermometers are not hard to find. Over the past few years I have acquired several different kinds.
Your average kitchen supply store and even some supermarkets will carry the Taylor thermometer, which most people probably had in their house growing up. Mounted on a flat steel blade, it clips to the side of the pan and usually has an alcohol or mercury based thermometer. If you plan to temper your chocolate manually, you will also need a chocolate thermometer, which has a much lower range than a standard candy thermometer.
Troubleshooting bubbles in your thermometer
If you drop your thermometer and it doesn't break, there is a good chance that there are bubbles inside the column. You must get rid of these since as the fluid heats the bubbles will expand proportional to the heat and your readings will be inaccurate. I've tried swinging the thermometer around on the end of a string and repeated boiling and cooling cycles with very little luck. I searched around on the web and found a suggestion for lightly tapping the levels back down. To remedy a dropped thermometer you have to drop the thermometer again—but in a controlled setting.
Place a thin dish towel over a hardcover book on your countertop. This will ensure a firm surface while minimizing any chance of breakage. Hold the thermometer vertically (as you would normally use it—bulb side down) several inches above the towel and with your other hand loosely secure the lower portion of the thermometer between thumb and forefinger (make an "OK" sign). When the thermometer is resting on the towel your guide hand should be high enough to keep the thermometer from falling over. Raise the thermometer 3 to 4 inches above the surface and let it drop to the towel covered book.
The thermometer will bounce a little on your padded surface and the bubbles should start to migrate upwards. After 5 to 10 minutes of this gentle dropping the bubbles should have worked their way to the top. Test the thermometer in some boiling water for both accuracy and to see if any small bubbles you didn't see become visible.
Digital thermometers—test them!
I also have a digital thermometer sold by Williams Sonoma that has a configurable alarm and also can clip to the side of the pan. I also recently bought a CDN digital thermometer on the recommendation of Cook's Illustrated. To be fair to Cook's Illustrated, the thermometer they really love is not the CDN model, but I didn't want to drop $85 (USD) on a ThermoWorks Thermapen.
Neither the Williams Sonoma or CFN digital thermometer seemed be consistent in their readings, especially compared with several of my alchohol based thermometers. It is OK if the thermometer is not 100% accurate, as you can and should expect to make adjustments to candy recipes, but the thermometer needs to be consistent so you can expect the same results on future batches. If it reads 208 in boiling water every time you test it, that should work. Of course, both consistency and accuracy are preferred.
Have a a favorite alcohol based thermometer? Buy two.
The first thermometer I had that I really liked was suspended in a neat stainless steel cage. It was in farenheit and had labels for various sugar stages (soft ball, hard ball, thread, etc..). The glass thermometer would slide out the top of the cage and you could clean each. It broke while I was cleaning it and I called around to find a store that had one. I drove 90 minutes round trip to Santa Cruz to get it. When I got there, it wasn't the same one I had bought their before, but this is my new favorite. Years later I found one similar to my original favorite, but I continue to use the replacement one, since I had already converted recipes to Celsius and made adjustments to the recipes in Celsius.
The moral of the story is: when you find a non-digital thermometer you like, buy a spare. They are easy to break and making a run to the specialty store you bought it at (if it wasn't purchased online) can be a hassle. If you prefer a digital thermometer then just make sure you have a spare battery handy. Some digital models require special watch batteries that could be hard to find at night.