Soft Schools Online Math Games
Soft Schools - Free online math games: Operations, Place Value, Number Sense, Estimation, Fractions, Decimals and Geometry.
Covers several Common Core Standards, but more importantly it builds the foundation for all the CCSS.
CCSS.Math.Content.4.OA.B.4
CCSS.Math.Content.1.OA.C.6
CCSS.Math.Content.2.OA.B.2
CCSS.Math.Content.3.OA.C.7
CCSS.Math.Content.3.NBT.A.1
CCSS.Math.Content.4.OA.B.4
CCSS.Math.Content.4.NF.B.3c
CCSS.Math.Content.4.NBT.A.3
CCSS.Math.Content.5.NBT.A.3