Although I've been dabbling with Open Street Map for a while I've only just discovered CloudMade whose fantastic style editor makes mapping a lot more fun and whose API is just gagging for creative ideas.
With the style editor I could easily create a map I've been trying to make for ages. I wanted to see just the green bits of London without the distracting other stuff. This is what it looks like. Have a look at the interactive version:
maps.cloudmade.com/?lat=51.518037&lng=-0.154152&z...