Some Useful Web Based Tools for Math for Elementary Education

Conceptua math: fraction tools

Thinking blocks: Modeling word problems and operations with fractions, ratios, and whole numbers

Math Playground tools including Cuisenaire rods,  pattern blocks, geoboard, a function machine, transformations, a pretty good spinner, fraction-decimal-percent scales, an algebra balance scale, transformation tool, long division... rather a lot of useful stuff!

Shodor interactive

Animal Rescue is pretty good for negative number visualization:

The Math Cats tessellation pictures are kind of fun
This looks like a useful list too:

I also like the games at Manga High:

and Lure of the Labyrinth: