Google Calculator is, perhaps, man’s greatest accomplishments that nobody really recognizes. It’s great for simple math, percent chance and the like, but it also does amazing unit conversions.
Sure, it has the standards you’d expect: Imperial to metric. Metric prefixes. Currency (for when you want to be sad) and those esoteric number bases that programmers love.
But also, it does Arabic to Roman numeral conversions. And vice versa.