NCTM's Illuminations site includes Calculation Nation. Students may play the games to challenge themselves, playing against the computer, or they may challenge another online player.
The games practice multiplication facts, factors, prime numbers, area, perimeter and other upper elementary through middle school concepts and skills. All games add an element of strategy so that students are prompted to try several possible moves before selecting the best move that garners the most points. Students won't realize they're really doing math as they PLAY these games and they'll keep playing to try to improve their scores and beat the computer or their online opponent.
Add this free resource to your online sites and encourage students to practice regularly. They may enjoy playing each other, parents, older siblings or students from other schools.