Books:
Bedtime Math series - short stories with math questions at a variety of difficulty levels
Which One Doesn't Belong - Book

Websites:
Helping with Homework - Tips and videos of strategies to use when helping with homework
YouCubed
Which One Doesn't Belong - Website with collections of four items (of mathematical nature).  An argument for selecting any one of the four items can be made. 
Videos of math strategies that promote student understanding - Short videos explaining what your children are learning in math, sorted by grade level
Bedtime Math
The Math Garden - Worksheets to improve your child's arithmetic skills
Talking Math With Your Kids - Ideas to incorporate math into everyday life
Figure This - Math challenges for families
Becoming a Math Family - Ideas for incorporating math into the lives of 3-6 year olds
Solve Me Puzzles - Logic puzzles
Math Before Bed - Discussion Prompts

iPad Apps:
Hungry Fish - Addition and subtraction practice
Wings - Multiplication practice