A drop checker is basically a CO2 indicator that changes color depending on the levels of CO2 in your tank...pretty good way of keeping track of your CO2 without having to constanly check your kH and pH levels.
I'm selling AquaticMagic glass drop checkers. Check my FS thread in the Swap n Shop forum. I think BlackSunshine is selling DIY acrylic drop checkers, but I'm not sure if he's shipping to Canada.
Haven't actually gotten around to deploying one yet, but the concept is that you create a mixture with a known KH state (using distilled water and baking soda) so that when it turns green you have your 'desired' CO2 PPM state. As there are lots of things in a tank that can throw off the standard CO2/KH/ppm chart (which I've wrestled with for years), this small container creates a fixed state where you can measure the CO2.
Folks adjust the KH level so that the green state of the drop checker indicates different CO2 PPM levels. If I recall correctly, with a KH=3 green implies CO2=30 PPM. If KH=4, then green implies CO2=45 PPM.