Sumdog is a free math website. Teachers can sign up in order to create individual student logins.
Sumdog is self paced learning aimed for ages 6-14 with 10 levels of difficulty. They need to answer math questions to progress to the next stage. Teachers can set the challenges for their students. Students can play individually or they can play with other users around the world.
http://www.sumdog.com/en/Introduction/
Thanks Anna.