In honor of it being December already, I'm adding my annual candy post. Tomorrow (I hope) I'll post a great peanut brittle recipe, but in the meantime, we're going to discuss candy thermometers. Your thermometer can be one of the most important pieces of equipment you own, especially if you make candy infrequently. A quality, accurate thermometer can make the difference between a mess of syrup and the candy you meant to make.
That said, there are some really lousy candy thermometers out there. I'd steer clear of those glass tubes with the colored plastic caps. Water gets in them and then you can't read the temp. They also aren't terribly accurate. Just don't buy a thermometer at a grocery store. All they ever seem to have are those cheap Good Cooks brand ones.
I have one very similar to this one: CDN Thermometer I have had mine for 12 years. It's almost indestructible because of the stainless steel back. It protects the glass and it keeps the thermometer off the bottom of the pan. I also love how long the thermometer is. It makes it so easy to read 1-2 degree changes in the syrup.
My mother used one of these for years: Sunbeam thermometer You'll notice that the glass tube is sealed so it's accurate and moisture can't get inside. They are very, very fragile however, especially when hot. You'll want to keep it well protected or buy a spare.
Instant Read Style You could choose one of these, it would probably read accurately and doesn't have mercury, but you'll notice the small amount of swing on the needle. This would make it very difficult to see small adjustments in temp. There's a big difference in 234 and 238 with candy syrups. If you can't see that difference on the thermometer, you don't want it.
Digital candy thermometers also exist out there, but I have no experience with them. If you do, please leave a comment. I'm very interested and I'd love to know if they live up to the hype.
Try going to a cooking supply store to find thermometers. You'll be able to ask questions and look at several different styles. They'll also have a better quality product. If you have a restaurant supply store in your area, try there first.
You will still need to test your thermometer before you use it no matter which one you choose. Have fun making candy! It's one of my favorite things about the holidays.