This award-winning website from Channel 4. Anyone can submit a 4 minute documentary to be included on the website: full instructions on how to do this are given, alongside access to a 'rushes' library of copyright-cleared music and video clips that you can incorporate into your documentary if you wish.
The site contains numerous films covering every subject imaginable - you can browse via title or theme and watch the documentaries online using standard viewing software, though you need to register with the site to be able to upload material. The website also includes a blog, details on documentary film-makers, a discussion forum, a news page and an archive section. The latter contains information on the history of documentaries, but also access to an archive of full-lengh documentaries, dating from the early 1900's to the present, all of which can also be viewed online.