I know the thermopen crowd loves their thermopen. That's a fact. I brew with a guy who has one and there is a place for them. The only problem is it is only useful when the thing you want to measure is accessible. The problem is, I often want to have the lid on something and know the temperature. For that reason, these little things have been one of the single best beer equipment purchases I have made:
Similar to the dual one posted previously, this is a single version. I started out using them for brew-in-a-bag while mashing in the oven. I had the mash tun pot in the oven and these things let me know the exact temp of the mash without opening the door. Thinking out loud, while the dual one does seem cool, I often measure temps several feet apart. Really depends on your setup.
This is the probe you want: PTFE/FEP Tip Probes (k-type). make sure to get the teflon tip. They are rated to 482 deg F and are completely waterproof. I have even dropped one of the readers into the boil kettle and they still worked.