Some Useful Web Based Tools for Math for Elementary Education

Conceptua math: fraction tools
https://teach.conceptuamath.com/app/tool-library

Thinking blocks: Modeling word problems and operations with fractions, ratios, and whole numbers
http://www.mathplayground.com/thinkingblocks.html

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!
http://www.mathplayground.com/math_manipulatives.html

Shodor interactive
http://www.shodor.org/interactivate/activities/

Animal Rescue is pretty good for negative number visualization:
http://www.crickweb.co.uk/ks2numeracy.html

The Math Cats tessellation pictures are kind of fun
http://www.mathcats.com/explore.html
This looks like a useful list too:
http://mathcats.com/mathexplorations/

I also like the games at Manga High:  https://app.mangahigh.com/home/games

and Lure of the Labyrinth: https://labyrinth.thinkport.org/www/