Math games and practice -  There are many others, so this is just a sampling of ones used by some of our teachers. 

99 Math  Great practice of math facts and such

AAA Math  Free math practice, lessons and games

Fun4theBrain  Math games

Illuminations  NCTM - Lessons and games from the National Council of Teachers of Mathematics

Interactive Sites for Education  GREAT math games

Snappy words angle puzzle 

Mathopolis  A great resource to use as the kids come into class in the mornings, for a warm up, for early finishers or just for kids who love math.  You can't go wrong with the questions or quizzes.

Number Fun: Counting Activities for Kids  Links to multiple games that have been filtered by The University of Alabama Birmingham

J-lab SOL  Practice site, all SOL subjects 

Instructional or informative -  These websites offer instruction or information to improve understanding, or go beyond what you might see in the classroom. 

CK-12- This is an extensive site with lots of resources on just about any subject. This includes free online textbooks, instructional games, interactive learning, etc.  

Khan Academy  Step by step instructional videos, practice problems, middle school to College!

Bedtime Math  This website gives many ways to make mathematics interesting and engaging at home

Desmos  is the website for the different calculators that are on the SOL tests.

Polypad  is an online manipulative program that can be used to show many relationships

.3 Blue 1 Brown  Visualizations of mathematics; really cool, but can be confusing! Highly recommended for students with a deep interest in mathematics. 

Math is Fun A kind of online mathematical dictionary written in clear and generally student friendly language 

Teachable Moments for Parents  Scholastic Channel on YouTube

VDOE website  Virginia Department of education website