Since 2010, Ordnance Survey has made part of its mapping data available under a free "OpenData" licence.
OS offers free and paid access to mapping material for the United Kingdom. You can access free OS mapping through their website or you can browse our blog post about the OS's free maps.
Several applications have been made available to assist OpenStreetMappers in making use of this data but before using them take a look at the following:
OS MasterMap can be previewed for free on our website after signing up for an account. Free previews are available with a 'pay-as-you-go' account and you get more when signing up to our professional account.
The National Library of Scotland's website offers free views of historical mapping provided by the Ordnance Survey dating back to 1840. You can access their website here.