Water freezes at 32 degrees F, and 0 degrees C.
Water boils at 212 degrees F, and 100 degrees C.
One inch = 25.4mm
One liter = 33.8 fluid ounces
One gallon = 3.8 liters (according to the top of a urinal)
That's all I really need to remember.
Sometime ago, a classmate asked me how many grams were in a Lund (I have no idea why they needed to know). I said, "453.5924."
Yeah, I know that to 4 decimal places. Had to convert it for years and years at work and it was easier to memorize than look it up all the time.