Moshi Monsters is popular with young children and the creators of this website have also created a free site for maths and english or literacy and numeracy depending on what you prefer. Much like mathletics, it allows children to compete against each other. I suspect it is good for kids up until about grade 4 who are achieving good grades in school. You can try it out at http://tutpup.com/ .

The other free website I came across is a story creating site that allows you to create a comic book story. The focus is on myths and legends. The resultant file can be played on an interactive whiteboard or on a computer.

http://myths.e2bn.org/create/

E2BN is the Learning Grid for the East of England and regional provider of the National Education Network. E2BN supplies Schools with broadband services and innovative online learning projects.