Visnos Mathematical Demonstrations
Visnos is a nifty site with many interactive tools for teaching and learning math. It tackles everything from basic addition and subtraction to fractals to the Sieve of Eratosthenes.
PhET Interactive Simulations
Many of the scientific simulations on the PhET site (from the University of Colorado, Boulder) are now able to be utilized on Chromebooks. They are no longer flash-based simulations.
PhET offers simulations from elementary level through to university.