Refractometer RF15 with Automatic Temperature Compensation (0-32 Brix)
Honestly, I never thought I would be buying a refractometer for brewing, but I'm glad I did. The speed at which measurements can be made takes the chore out of gravity measurements and the small sample size required means that I can run more fermentation experiments.
The accuracy is much better than I thought it would be. The resolution of the scale is 0.2°P which is better than my hydrometer. In a side by side comparison the overall measurement accuracy was about the same as a hydrometer. (Post detailing this later)