A good instant-read thermometer is one tool that is really great to have around the house. I can't tell you the number of times it has saved me from serving undercooked meat or chicken. When choosing a thermometer you want to make sure you get one that is easy to read, has a great response time, is accurate (this should be priority one), and has a well thought-out design.
The thermometer that gets all the praise (you'll see the chefs on America's Test Kitchen touting this one, as well as Alton Brown) is the Thermapen thermometer. It's well known for its super-fast speed and accuracy, plus the probe portion is quite long (you'll be able to get to the true center of a pot roast) and conveniently folds in. It's available in several different colors, however it doesn't come cheap. This guy is the king of thermometers and its price tag shows: $84.95.
Two more that won't break the bank, including one that will let you know when things are done, so
The Taylor High-Temp Thermometer is a reliable instant-read thermometer that should last for years. The large digital face allows for easy reading, and it has a switch that will convert from C to F. It also has a wide temperature range (-58F to 500F) and is available for $12.99.
Finally, if you're working on a large pot roast, or whole chicken, it's really great to know when something is done. If you constantly have to open the oven door to check, you're letting all that warm air out. To prevent this problem try a programmable cabled thermometer. This one by CDN has a thermometer portion that is on a stainless steel cable and can withstand high heats (just don't put the tip directly on a hot surface). Stick the thermometer into the roast, trail the cable out the door, close the oven and plug the thermometer into the base. The base, has a large digital display and can be programmed to beep once it hits a certain temperature. Now you really can set it and forget it! This model is available for $29.95 and comes with a 5-year warranty.