Fact Dash - Addition and Subtraction Fact Practice

MAP Math Activities by RIT Score
Doubles to 20 - Addition Practice
Addition/Subtraction Practice Sites:
This site has many, many activities to develop understanding of many basic skills (place value, ordering, time, measurement, etc.)
Peninsula School District - This site has many great supplemental links. It is designed for K-2nd grade.
This is a math dictionary for kids.
This site is a work in progress! I'd love to review your favorite math sites, so send them in. More sites will be coming soon...