Originally Posted by Janx
...I recommend welding a threaded nipple into your kettle and getting one of those dial thermometers with a metal probe that screws into the nipple....
As far as fermenter temps, I don't bother personally, but they make those little sticker type thermometers that go on the side of the carboy (the outside...I'd never risk your beer by sticking a thermometer into it.) The best way to control temps in fermentors is to control the environment they are in. Build a cool box, put a thermostat on a chest freezer...something like that. That's your best bet by far.
Couldn't have said it better. The Polder thermometer probes are extreeeeeeeemly delicate. I've toasted 2 myself, and that was just for regular kichen cooking. The bi-metal type (the metal probe Janx mentioned) is very durable and fairly quick reading. You can even calibrate them if they start to wander a bit. Check out www.stpats.com
under Test Equipment/Thermometers and scroll down to the bi-metal ones. This is the cheapest I've seen these. If you want something a little less "permanent", try this: http://www.williamsbrewing.com/AB1605000/showdetl.cfm?&DID=7&Product_ID=546&CATID=18
This is a great little jewel to have around the brewery. I even bought an extra one for the kitchen.
And again, Janx nailed it on the ferment temperature. Don't put a thermometer in your beer! I've had good luck with the stick on type thermometers. If you brew infrequently, or have varying ambient conditions in your fermenter space, this can be a real help to keep an eye on your fermenter temperature.