National Library of Scotland Maps API
The National Library of Scotland has released an API to give developers access to a map of the UK using out of copyright Ordnance Survey maps, dating from the 1920s to the 1940s.
The API uses Google Maps to create an interactive map based on tiles created from the Ordnance Survey Maps. You can see the basic map here. This map can be embedded into other websites and blogs just by copying and pasting the iframe code. You can even embed a map using a dynamic URL so that the embedded map is zoomed and centred on your chosen location.
However you can also use the API to create your own mashups. For example here is a map that has an added transparency slider, that allows you to view the Google Map tiles beneath the OS maps.