The Art of Problem Solving contains problems from international competitions, undergraduate competitions, and national and regional competitions. There are solutions for about 5% of the problems.
EqWorld covers some the most important mathematical equations and solutions in science. It contains information on algebraic equations, ordinary differential equations, partial differential equations, integral equations, and functional equations.
QuickMath is free online calculator that solves equations and many types of algebra and calculus problems.
An online program that instantly solves a wide variety of mathematical problems; capable of handling derivatives, trigonometric expressions, & even word problems.