Subcategories:
Measurements 
Measurement conversion tables and information about various types of measurements and measurement systems.
Convert length, area, volume, temperature, and weight. Also features an online unit dictionary, giving definitions for the most commonly used Metric and Imperial units.
http://www.metric-conversion.net/
Conversion tools to convert between various metric units of measurements. Includes distance, area, weight, volume, temperature, speed, astronomical, and binary digit converters.
http://www.metricmetric.com/
Unit converters of 99.9% known units in 67 categories.
http://www.unitconversion.org/