Any list of essential kitchen tools must include some way to measure temperature. There are three basic types of kitchen thermometers, the simple dial type that are places inside the oven, infrared thermometers and probe thermometers.
In our opinion, you should have all three. Ovens temp dials, especially older ones are not guaranteed to be accurate. Placing an inexpensive dial thermometer inside will give you an accurate temperature reading.
Infrared thermometers read the temperature of whatever they are pointed at. They are great for quickly checking the temperature of a hot skillet or making sure your refer is cold enough.
Probe thermometers are for checking the internal temperature of meats or anything else where internal temperature needs to be monitored.
None of these are expensive. The infrared ones are around $20 (although there are much more expensive ones) and the others can be had for under $10.