OS MasterMap Viewer is a Qt4 application capable of reading Ordnance Survey MasterMap XML files (or compressed XML) and displaying their contents.
Maps are rendered using OpenGL which allows smooth scrolling and zooming up to reasonable scales. Maintaining performance at low zoom levels is a work in progress.
PDFs of parts of the map up to any size can be exported.
Most features of the topography layer are supported. Exceptions include patterned fills and boundary line objects.
Example files can be downloaded here.
These details are provided for information only. No information here is legal advice and should not be used as such.