There are an awful lot of math sites on the internet and too often, they are filled with distracting ads that make it difficult to find the learning material, or too many games that don’t so much teach math skills as babysit kids.
Here are some I’ve found useful in my technology lab. They’re straightforward, with an uncluttered interface (mostly) and a focus on teaching not entertaining:
Wild on Math—simple to use, challenging for the best of students
Math and Virtual Manipulatives and Tessellations –well organized, using a whiteboard with shapes and colors as the virtual manipulatives.
Math website—popular, a standard 
Math—by Grade Level–a longtime favorite that reinforces basics, math facts and speed math
Math–Mental Math –traditional mental math practice. Well done.
Math–Minute Math –Mad Minutes
Games that make you think –logic, for K-3
Multiplication.com–lots of multiplication problems, lessons, games, with a few on addition and subtraction
Interactive Math Lessons–lots of them. You’ll find everything you need here.
Math problems by skill and grade
