Developer website: Alegre Software
MetricMaster is a measurement unit conversions calculator containing over 2,000 metric/S.I., imperial, U.S., and other units in its database, most to 14 significant digit precision. New units can be easily added by the user. MM Supports ft/in/fractions, scientific notation and control of significant digits. MM can also convert currencies, clock times, and many physical constants. Version 3.0 has been given an elegant aqua style interface and now contains 600 definitions for units and unit groups (in case you didn't know what a weber was). MM also contains extensive information on the history and development of modern measurement systems.
Mac OS 8.6 or later