MrCalculator
Features calculators for arithmetic operations, hexidecimal conversions, camera focus evaluations, finding monthly mortgage payments, color codes, and to make sundials.
http://www.mrcalculator.com/

Physics Calculator
This RPN calculator implements arithmetic and scientific operations and functions, as well as predefined physical constants.
http://www.physique.fundp.ac.be/Comp/PhysComp.html

ABE Volume Calculator Page
A collection of volumetric calculators for a variety of shapes. Including cylinder sphere barrel pyramid and dome.
http://grapevine.abe.msstate.edu/~fto/tools/vol/index.html

Conversion of Units
Database of conversion factors and a calculator.
http://www.chemie.fu-berlin.de/chemistry/general/units_en.html

Dario Alpern's Complex Number Calculator
Computes basic operations, 1/z, pi, exponent, log, trig, and polygamma functions. Includes memory feature.
http://www.alpertron.com.ar/CALC.HTM

Calculate for Free
Performs basic math operations, as well as square root and percentages. Includes memory save and recall.
http://www.calculateforfree.com/

Calculator City
Includes tools to solve equations like for velocity, density, financial, geometry, algebra, calculus, astronomy, chemistry, physics and webpage color coders. Also, has trivia fact pages, such as state demographics and presidents.
http://www.1728.com/

Calculator.com
Online calculators for a variety of conversions and calculations.
http://www.calculator.com/

Martindale's Calculators
A collection of calculators for the sciences, business and everyday measurements. Organized by subject area.
http://www.martindalecenter.com/Calculators.html

Hiox Calculator & Converter
Features calcutors for arithmetic functions, scientific computations, binary conversions, areas, finanical interest, prime numbers, recurring fractions, factoring, matrices, HEX color converter, date and day calculations, as well as for measuring bandwidth.
http://www.easycalculation.com/

ScatterMall.com
Free calculators for science, travel, students, health, education, weather, math, and business. Includes graphing calculators and measurement conversions.
http://scattermall.com/freetools/

Percent Change Calculator
Measures the difference in value using percentages. Includes bar graphs with the answer.
http://www.percent-change.com/