Wine and beer hydrometers are used to measure the density or specific gravity of liquids as compared to pure water. Because density changes as temperature changes, a useful tool to have in your arsenal is a thermohydrometer like those found here. You'll also find a good assortment of single- and dual-purpose hydrometers for measuring the specific gravity of your wine or beer.
For monitoring boil temperatures, nothing is better than the large-dial thermometer with a 12-inch long probe. It will clip right on to the side of your brew kettle and, with its large, easy to read numbers, allow you to accurately monitor your boiling temperatures. It's also useful for keeping track of how quickly your wort is cooling. Another handy item featured on this page is the refrigerator thermostat. It has digital accuracy and lets you turn a standard chest freezer into an awesome, beer-storage kegerator!