IXL - IXL has many practice exercises for an extensive list of math standards.
Fact Monster - This website also enables the students the opportunity to play math games. There are games for addition, subtraction, tens/ones, ordinal numbers, and much, much more.
Illuminations - This website was designed by NCTM (National Council of Teachers of Mathematics. It has an amazing range of activities across the entire curriculum from Number Sense, Geometry, Probability, Equality, etc. It also provides K-6 games.