Hanna Instruments Checktemp 1.
I have bought 4 digital thermometers over the past 2-3 years. Each time paying about £10-£15. Each time I have been disappointed with their accuracy, especially after a bit of use. Finally, I decided to
invest in a Checktemp 1 and have not been disappointed. I used it to check each of my other thermometers. Only one (
here) was accurate (to +/- 1C) - 2 were out by between 2 and 3 degrees (depending on the temperature being measured). When you consider that we sometimes want to mash at 65 rather than 66 to get a drier beer, these inaccuracies are crucial.
Compared to others it's a lot of money (£45 new), although I'm sure Andy will find it on Ebay for a fiver
HTH