Tutpup is a free online resource for children to play simple maths and English games against other children from all around the world. The beauty of the site is that every child chooses a nickname, based on a colour, animal and number, and therefore remain completely anonymous. A teacher can sign up and create a class code, so that their students only need to enter a minimum of information on the site.
Games currently on the site involve addition, subtraction, multiplication, division, algebra and spelling. There is a Hall of Fame which shows the members who have won the most games over the last 24 hours, 7 days or month, with the leaders having won over 1000 games in the last 30 days! It seems to be a really motivational site for students to practise key skills.
(Thanks to Tom Barrett for pointing me there.)