Apex
My dad built Apex in the 1990s and released it as Windows 3.1 shareware. It put everyday conversions into one compact window, with vivid and lifelike 256-color icons. Way ahead of its time!
Choose a category, select two units, and type into either field. Apex immediately recomputes the other value.
About the recreation
The calculations are data-driven TypeScript, and each category has recreated indexed 256-color pixel art.
The lightning bolt still belongs in that blue title bar.