Create a Graph
Greg Tang Math
2048 - fun math strategy game
Flappy 2048 - a combination of Flappy Bird and 2048
Image Chart Editor by Google - create charts and graphs and embed onto your wiki, blog, etc.
12 Best Math Apps and Websites Aligned to Common Core Standards - via Graphite (August 16, 2013)
XtraMath - Practice those math facts! Used by SMMA 1st Graders.
Math Game Time - grades PreK-7, includes math how-to videos
Everyday Mathematics
A+ Math
The Playground of Mathematics - addition, subtraction, multiplication and division practice
Math Cats
Teach R Kids Math
Ask Dr. Math
Multiplicaton Game
Math Games - Math Magician and many others!
AAA Math
Cool Math
Count on Math
Figure This!
National Library of Virtual Manipulatives
Sudoku - online and in a variety of difficulties
Johnnie's Math Page - tools and activities
Algebra Resources - tools and resources for students and teachers
Polygon Sorting - interactive
Triangle Sort - interactive
Mr. Duey's Fun Zone - middle school math games
Brainormous - math fact game that reports progress in printable form
A Maths Dictionary for Kids - Animated, interactive definitions, examples, activities, practice and calculators
I Play Math Games - lots of games for all ages
Math Zapper - fractions, decimals and percents
Weblist - Great interactive math games for grades 3-5
Learn Your Tables - multiplication facts practice
Math Games by Jefferson Labs - middle school
Stay or Switch? - fun game of probability. Which of the three doors is the car behind?
Symbaloo - Lots and lots of interactive math games
Fun 4 the Brain - addition
Fun 4 the Brain - subtraction
Fun 4 the Brain - multiplication
Fun 4 the Brain - division
Hooda Math - Grades K-8: Lots of fun games to help learn math facts, fractions, integers, and algebra
Analog Clock

For Teachers:
Let's Play Math! - Math resources and games from preschool to pre-calculus.
Math Mama Writes - Many resources for students from PreK-College.
9 Amazing Math Websites You Should Explore - blog post