Here is a list of highly recommended websites and apps where students can build number sense:
http://motionmathgames.com/
http://www.buzzmath.com/
http://www.conceptuamath.com/
http://www.dragonboxapp.com/
http://www.brainpop.com/educators/community/bp-game/refraction-2/
This one requires a school to have a license: http://web.stmath.com/