National Council of Teachers of Mathematics
The Math Forum: Intro to Measurement for Primary Students
This site from the Math Forum is a great introduction to measurement for primary students. Each measurement activity has four components: interactive, manipulative-based project; technology; paper/pencil practice; literature connection.
Math Playground
Math Playground: Math Bars
Create bars whose length and color represents a one-digit number. The bars can then be manipulated to visualize basic math operations.