PlanetCalc

PlanetCalc is a free online collection of calculators covering everyday, professional and academic tasks. The site hosts close to a thousand calculators in categories such as calendars, dates and time, health, engineering, computing and mathematics, and lets registered users request new ones or embed existing calculators on their own sites. It is available in several languages, including English, Russian, German, Spanish, French, Portuguese and Chinese, and is maintained by a community of contributing authors.