Converters
Units in, units out. Weight, length, temperature, volume, speed and the odd historical one.
Nearly every tool here does the same arithmetic, and they differ only in how much of it you are left to decide for yourself. A named pair — kilograms to pounds, Celsius to Fahrenheit — opens with both ends already chosen and prints the factor under the answer. A family converter opens with a picker instead and covers everything in its dimension.
Named pair, or the whole family
Reach for the named pair when you know both units and want the number now: one field, one answer, and a reference table of the values either side of yours. Reach for the length, weight or general unit converter when a single job moves between several units — switching inside one page beats opening four tabs. Height is split out from length on purpose, because the answer people want back is feet and inches rather than a decimal.
Roman numerals sit here on a technicality. It is a notation rather than a unit, but it is still a two-way conversion with a table behind it. What is deliberately elsewhere: cups to grams, because a cup of flour and a cup of honey are the same volume and very different weights, so the answer has to be a per-ingredient lookup rather than a factor; anything measured in hours or days, which is Date & time; and currency, which needs a live rate and would be stale the moment it was cached.