Numbers
This site from the BBC has lots of activities for students to practice their number skills! They can practice their times tables, learn how to write big numbers, round numbers, learn about negative numbers, learn about place values and more with fact sheets, games, worksheets, and quizzes.
Intended Audience: Students Reading Level: Elementary/Middle School Teacher Section: No

AAA Math
Practice your addition, subtraction, multiplication, and division skills at this site from AAA Math. You find lots of explanations, interactive practices and challenge games that range from basic facts to working with fractions and positive and negative numbers. You can locate skill sets by topic of grade level.
Intended Audience: Students Reading Level: Elementary/Middle School Teacher Section: No

A+ Math
You can practice math skills with games and flashcards at this site and you can even make your own flashcards and worksheets.
Intended Audience: Students Reading Level: Elementary/Middle School Teacher Section: No

Count Hoot
Play addition and subtraction games with Count Hoot at this site from the BBC.
Intended Audience: Students Reading Level: Elementary School Teacher Section: Yes

Sum Sense
Drag and drop numbers into an addition equation as quickly as you can!
Intended Audience: Students Reading Level: Elementary School Teacher Section: No

SpeedMath - Addition and Subtraction
Create an equation, as quickly as possible, from the four digits the computer gives you at this site from Jefferson Lab! You might also want to try: SpeedMath - Inequalities , SpeedMath - Multiplication and Division , SpeedMath Deluxe , and the Place Value Game.
Intended Audience: Students Reading Level: Varies Teacher Section: No

EdHelper
You'll find a large collection of free printable worksheets for grades K-12 at this site. You can also become a paying member of the site and create customized worksheets.
Intended Audience: Teachers Reading Level: Elementary/Middle School Teacher Section: N/A

Dositey
You'll find interactive math games and printable worksheets for grades K-8 at this site.
Intended Audience: Students Reading Level: Elementary/Middle School Teacher Section: No