Useful and accurate mapping websites: United Kingdom What are your favourite regional online websites or blogs relating to mapping or routing that you find useful and accurate. 

See also: What are the most useful or interesting cycling-related websites or blogs?

Please vote up or down based on the accuracy/usability of these services and post one website link at a time.
CycleStreets
CycleStreets is a UK-wide cycle journey planner system, which lets you plan routes from A to B by bike. It is designed by cyclists, for cyclists, and caters for the needs of both confident and less confident cyclists.
The service looks to be free, however, funding issues are causing the site's development to be slow, and they accept donations.
The site is currently in beta, and has gaps in coverage. There's an iPhone app for the site, and an Android app was in development last year.
The most useful feature, the Journey planner provides information you would normally get from any free mapping service, turn-by-turn directions, journey time and distance.
It also gauges the quietness of the route based on how much of the journey is based on cycle paths and roads, provides an elevation profile and the ability to import your journey to Google Earth or export it to a GPS device