There are two types of oven thermometers: one with a dial face and the other with a glass bulb (for safety reasons, the bulb is filled with colored alcohol rather than mercury). We prefer the readability of dial-face thermometers; Cook's Illustrated also "found that the tinted alcohol used in bulb thermometers can get stuck, compromising accuracy." In either case, when choosing a thermometer, look for one that's sturdy and easy to read.
Different models can stand, clip, or hang on oven racks. In considering our next thermometer purchase, we're looking for one that would be easy to move and read as we use different rack configurations and baking pans. We also want a thermometer with both Fahrenheit and Celsius temperature scales; however, unless you frequently use non-US recipes, it's best to forgo this feature as the less cluttered the face, the easier it is to read.
We're currently leaning towards the Cooper-Atkins Oven Thermometer (about $7), which Cook's Illustrated recently recommended, but would love to hear our readers' suggestions, too!
• See the Cook's Illustrated reviews: Oven Thermometers