Mathway is a topic specific free online graphing calculator. They are a great resource for displaying graphs, computing derivatives/integrals, as well checking your answer!
Desmos is an easy and free to use graphing calculator. Graph cartesian and polar plots, conic sections, probability plots, derivatives, integrals, areas and Taylor Series expansions!
GeoGebra is another easy to use graphing calculator, but can do more than graph. Here they also can plot in 3D, construct geometric objects, and display Gaussian probability curves!
Math Portal provides Math calculators, formulas, tests and lessons for Algebra, Geometry, Calculus, and Linear Algebra. Use this site to check your work and ensure your answer is correct!
Wolfram Alpha is as close as you get to computer aided algebra software, which is not software! On top of algebra and graphing, they feature explanations, diagrams, and much more!
The Geometer’s Sketchpad® software is for teaching and learning mathematics. Sketchpad® gives students visual way to learn mathematics that increases their attention to detail.
MacTutor is an online resource containing biographies of nearly 3000 mathematicians and over 2000 pages of supporting materials. The site is supported by the University of St. Andrews in Scotland.
Plus+ Magazine is a fun resource for Math educators and students to stay up to date in the world of Mathematics! They also feature interviews with industry professionals and mini games!