I bought this thermometer from Target for about $17, which has a remote stainless steel probe on a wire a few feet long.  It's digital, and is supposed to be 'instant-read'.  In addition to being a thermometer, it's also a timer, and also has the ability to sound an alarm when a target temperature is reached, all of which makes it pretty handy for brewing purposes.  To top it all off, it's got a magnetic base.

Waterproofing

I heard that getting liquid inside the probe would render it useless - not surprising.  The probe itself is all stainless with no gaps or holes, but where the wire enters it it is not entirely sealed - after all, it's meant for barbecue and oven cooking, not immersion in liquids.

I used a couple feet of silicone aquarium air line tubing.  Silicone is good for high temperatures - this particular tubing didn't have any specs, but I assume it's fine for boiling wort and experience hasn't shown otherwise so far.  I had to cut the wire to get the tubing on - trying to slide it all the way over the probe would have been near impossible.  Getting the tubing on was still a hassle, as the braid on the wire acted like a chinese finger trap, only inside out - it kept expanding in spots and lodging itself in the tubing.  I poured some rubbing alcohol down the tube to act as a minor lubricant that would evaporate quickly, and just kept massaging the tube as I slowly twisted the tubing down onto the wire until it was through.  I slipped the tubing about an inch onto the back end of the probe, and put a small zip-tie on there.  The tubing fits the probe quite tightly and should be relatively water-tight but hopefully the zip-tie will make it a sure thing. 

Soldering the wire back together was a bit of a hassle, because the wire is a single insulated wire with one inner conductor, sheathed in stainless steel braid.  Naturally, solder wouldn't stick to the stainless, so I soldered the inner conductor, covered it with heat shrink tubing, and then wrapped it with a piece of fine stranded bare wire, connecting the shielding of the two pieces.  Then one big piece of heat shrink covered the whole mess.

Usage

This thermometer is great for partial mashing.  Stick the probe in your pot while you heat the strike water, set the target temperature to your desired strike temperature, it beeps when it's ready.  Mash in, close the mash tun, stick the probe in, set the timer, and wait for it to beep again.  The probe quite conveniently fits through the pour spout in the lid of my insulated mash tun, so no additional holes were needed!  I did need to straighten the probe for this, because it came in an L shape.

Using the thermometer with the mash tun

My only real gripe with this thermometer is the accuracy.  It doesn't even display to one decimal place, and I don't think it's more than 1C accurate anyway (probably worse than that).  But for my purposes, it was well worth the money.