http://www.langexpress.com/Kana-Game.html
Quite fun.
I'm a complete beginner with Japanese, and even more so with kana. It seems to me that once you can write Japanese words in romaji then writing in hiragana is simply a matter of memorising the symbols and their corresponding sounds (at least I hope so).
The words they have on the game is quite limited, but another way I play is to decipher the hiragana I see on this website.