The Valve Archive is an amazing community driven repository of rare media covering the legacy of Valve, their games and related content. It is home to over 55GB (and growing) of data and is updated regularly by the fans behind it.
The site provides a clean and very easy system for browsing through the files and previewing contents.
The care and attention to detail is a tribute to those who have worked on the Archive.
You can find out here how to sync the data.