DVLA - The Driver and Vehicle Licensing Agency
The DVLA Web Site contains quite a lot of useful information, although it's still a little limited for Motor Trade purposes. However, for the general motorist, the site is a recommended reference as it covers topics such as legislation and licensing, together with details of changes recently being introduced to combat vehicle crime. Many documents and standard forms are now available for download as electronic Adobe Acrobat PDF files (which can save you a trip to the Post Office).
VOSA - The Vehicle and Operator Services Agency
VOSA is the merger of the Vehicle Inspectorate (VI) and the Traffic Area Network (TAN) division of the Department for Transport.