SpeedCrunch is a free, fast, high precision open source algebraic calculator with an intuitive interface and many features, such as variables, syntax highlighting, functions, persistent input history & more.
Qalculate! is a multi-purpose cross-platform desktop calculator. It is simple to use but provides power and versatility normally reserved for complicated math packages, as well as useful tools for everyday needs (such as currency conversion and percent calculation).
Wolfram Alpha is a "Computational Knowledge Engine" - different from a search engine, it answers factual queries directly by computing the answer from structured data, rather than providing a list of documents or web pages that might contain the answer.
Mathematica is a computational software program used widely in scientific, engineering, and mathematical fields and other areas of technical computing.
Online math solver, equation editor and database, graphing calculator and science/engineering problem helper.
A simple yet powerful calculator that includes standard, scientific, and programmer modes, as well as a unit converter. It's the perfect tool to add up a bill, convert measurements in a recipe or other project, or complete complex math, algebra, or geometry problems.