Axon game
Axon, a game about growing neurons

Wellcome: Axon Game.

Wellcome seems to have an interesting collection of educational web games. Axon, in particular, seems pretty fun to play (although my 5-year-old computer seems a bit too slow for it).

They have more on their main games page. I was impressed with how well produced the games were, they all look pretty good. And the educational aspect is mostly subtle, you can play them without reading more about the science, but the information is there if one is interested.