I've just entered Waterstone's What's Your Story? competition, where you have to write a short story with only 600 characters to fit onto a postcard. It was really difficult, but fun too! You also get to customise your postcard. Some of them are really good.
Anyway, you can read mine (easiest to find if you view by date in the Gallery section - mine was uploaded today, called 'Time Bandit'), you can read other submissions, and also enter yourself at: http://www.waterstoneswys.com/