eCalc offers two types of calculators...

• basic - this version works as a simple web-based alternative to calculators that you can find in pretty much any convenient store.
  Features...
  
  • Add,
  • subtract,
  • multiply,
  • divide,
  • square root, etc. and
• scientific - this version is for more advanced calculations and includes a unit converter
  Features...
  • Web based multi-functional calculator and unit converter.
  • Equation Solver,
  • Scientific Functions,
  • Complex Numbers,
  • Decimal to Fraction,
  • RPN or Algebraic.
  • Unit Converter for
    
    • Mass,
    • Time,
    • Power,
    • Volume,
    • Area,
    • Length,
    • Energy,
    • Temp,
    • Force and
    • Pressure units.

Free, no sign-up or registration needed.

Visit http://www.ecalc.com/.