I would really like to do what you can do with EveryTrail (see earlier posts) with OS mapping.
The Ordnance Survey have just announced the launch of OpenSource for developers. This looks like rather hard work unless you are used to Java programming.
Anquet tell me that they are soon to launch something similar to EveryTrail but with OS mapping so I will be interested to see how that pans out. However, there are licensing restrictions with showing OS maps on websites, so I will be getting in touch with OS to see exactly what the situation is. Watch this space.