...and now there's OpenOrienteeringMap. UK coverage only at the moment, and the tiles are generated on-the-fly (and cached) so might be a bit slow in places.
The styling, symbology and colours aim to match those in the ISOM spec (PDF).
I particularly like the way Milton Keynes has come out.
Anyone know of parks or other areas in the UK which have been OpenStreetMapped to incredible detail (e.g. individual prominent trees, different terrain types)? I would like to highlight these and see how they come on the map. BestOfOSM.org doesn't have much in the way of UK examples unfortunately.