I posted a link to this thermometer last night in a different thread :
Can't say enough good things about this one. It's affordable, NIST-traceable, and has a long stem (8") which reaches easily into the center of my mash cooler. I've checked it a few times against a laboratory reference thermometer and it stays within 1/2 a degree F. It sells on Amazon fairly often for even less.
EDIT - I bought the cheaper of the two in this link.
And I seconded this last night too! Quality piece of equipment!