Sumdog provides free numeracy games at 10 different levels. For a great way to practice adding and subtracting with negative numbers, play these games at level 10. There is a complete list of topics at each level here. Students can choose from several games.
I like the way that the various skill levels can be restricted; the site is aimed at students aged 6 to 14 (having said that some of my Year 11 (age 15-16) students looked like they were rather enjoying themselves recently) so I want my secondary age students to practise the skills at the upper end of the age range and have currently restricted them all to levels 8, 9 and 10. It is possible to set up competitions which I have done very successfully with Year 10 as one of our many Enrichment Week activities. See the Teachers’ page, also the help section for teachers.
Manga High seems well known for its games, it is also well worth investigating the excellent Prodigi quizzes available.
Hundreds of these are available offering excellent curriculum coverage. To access the resources select Challenges from the Activities menu.
The search facility offers teachers the opportunity to filter by curriculum area, age, level and whether a calculator is allowed; for example a simple search of Prodigi quizzes on Algebra gives the results as shown below. (A complete list of all quizzes is available here.)
Unlike the available games students cannot see the available Prodigi quizzes unless a teacher sets them as challenges (or you have a subscription). You can view very clear instructions on setting challenges on the Manga High website.