Sumdog - These are FREE games that make Math fun and competitive! Love it and most of al, my kids love it! Activities for all grades that follow the Common Core standards. You can create challenges between the students in your class, same grade, the entire school, or the world. Students get points. They even set up contests for each state.
MangaHigh - Another site with Math games. You have to create a school account so that students can play the game, but it's FREE. My students love it, and I am able to assign challenges on the skills I want to reinforce.
HoodaMath - FREE Games, tutorials, worksheet, movies, and a very, very, cool theme song! They are in the process of aligning their games with the Common Core standards. The kids have a lot of fun, while they learn.
TenMarks -FREE Online Math program that provides individualized practice for each student. Love the fact that I can create assignments using the given curriculum. They are aligned with the state and common core standards. Reports can also be sent to the parents. You can upgrade to a premium account if you wish.
Arcademic Skill Builders - Play games and learn! They have games for a variety of subjects, including Math. If you want reports, etc..., you have to upgrade.