Sounds like you're looking for something with a bit more sophistication than this, but I use a thermometer from stpats.com. It's digital readout and it screws into a 1/2" NPT fitting on my kettle. They're listing it for $38 now. Attach it, look at it, and get the temp. Cheap enough that you don't mind upgrading later.
Commercial on Tap: None
Drinking: 90-Minute Redhead
Secondary: Sweetwater 420 Clone
Primary: Spiced Winter Ale