Ordnance Survey has finally made available their mapping data as part of their OS OpenData initiative. You can view the detailed street mapping here, or download the data required to making your own maps or deploying them in various projects. Data includes 1:250,00 GB raster map, GB miniscale and OS Street View.
Sadly walkers hoping for free outdoor maps will find themselves disappointed as Ordnance Survey flagship Explorer and Landranger raster maps will not be made available. OpenData will also not include footpaths, bridalways and terrain date - all of which are crucial to walkers. For now, at least, walkers who wishes for a digital version of Explorer and Landranger will have to make do with Memory Map and others.