1. PRIMARY MUSIC RESOURCES

1.1. Streaming Audio
I have limited this list to the free and freemium services with a huge geographic footprint and huge collections. In most markets you'll find local and/or regional (mostly U.S.-oriented) alternatives (Pandora, Rdio, Rhapsody, Beats, Google Music, iTunes).

Spotify
Streaming music service.
Genres: All
Depth: 9
Breadth: 7
File Quality: 8
Accessibility: 8
Database: 6
Search Engine: 5
Writing: NA
Interface: 5
Accuracy: 6
Entertainment: 9
Regions: Global
Media: Audio
Period: 1920 - Now

Last.fm
Streaming music (audio & video) service.
Genres: All
Depth: 7
Breadth: 7
File Quality: 8
Accessibility: 8
Database: 6
Search Engine: 7
Writing: NA
Interface: 6
Accuracy: 7
Entertainment: 7
Regions: Global, UK
Media: Audio, Video
Period: 1920 - Now

Kboing TIP!
Surprisingly extensive streaming music resource. Includes many rare recordings.
Genres: All
Depth: 8
Breadth: 7
File Quality: 7
Accessibility: 8
Database: 8
Search Engine: 8
Writing: NA
Interface: 7
Accuracy: 7
Entertainment: 7
Regions: Global, Brazil
Media: Audio
Period: 1920 - Now


1.2. Video & Audio Sharing
Never trust uploaders' annotations or comments - just consider the resources listed below to be a starting point for further research. The rarest files tend to disappear from YouTube because the bigger rights holders police it intensively (but check the Russian and Asian klones to see if anything has escaped their attention there). Use the "Long (> 20 minutes)" filters to find full films, documentaries, concerts, albums more quickly.

YouTube
Global platform for discovering, watching and sharing video and audio.
Genres: All
Depth: 9
Breadth: 9
File Quality: 8
Accessibility: 9
Database: 6
Search Engine: 8
Writing: NA
Interface: 7
Accuracy: 6
Entertainment: 9
Regions: Global
Media: Video, Audio
Period: 1910 - Now

Mail.ru Video TIP!
Russian platform for discovering, watching and sharing video and audio. Often offers access to clips which were deleted from YouTube.
Genres: All
Depth: 8
Breadth: 8
File Quality: 7
Accessibility: 7
Database: 7
Search Engine: 7
Writing: NA
Interface: 6
Accuracy: 5
Entertainment: 8
Regions: Global, Russia
Media: Video, Audio
Period: 1920 - Now

Yandex
Russian platform for discovering, watching and sharing video and audio. Often offers access to clips which were deleted from YouTube.
Genres: All
Depth: 7
Breadth: 7
File Quality: 7
Accessibility: 7
Database: 7
Search Engine: 7
Writing: NA
Interface: 5
Accuracy: 5
Entertainment: 7
Regions: Global, Russia
Media: Video, Audio
Period: 1920 - Now

Nate
Some music that always disappears from YouTube tends to be kept available on this Korean klone.
Genres: All
Depth: 6
Breadth: 7
File Quality: 7
Accessibility: 7
Database: 6
Search Engine: 7
Writing: NA
Interface: 7
Accuracy: 6
Entertainment: 7
Regions: Global, Korea
Media: Video, Audio
Period: 1940 - Now

Soku
Some music that always disappears from YouTube tends to be kept available on this Chinese klone. They're not generous when it comes to sharing their bandwidth and server power with the West.
Genres: All
Depth: 7
Breadth: 7
File Quality: 7
Accessibility: 6
Database: 6
Search Engine: 7
Writing: NA
Interface: 7
Accuracy: 6
Entertainment: 7
Regions: Global, China
Media: Video, Audio
Period: 1940 - Now

Daily Motion
YouTube-like content with a European touch, but they've seen better times.
Genres: All
Depth: 6
Breadth: 7
File Quality: 7
Accessibility: 6
Database: 6
Search Engine: 7
Writing: NA
Interface: 7
Accuracy: 6
Entertainment: 7
Regions: Global, Europe, France
Media: Video, Audio
Period: 1940 - Now

Vimeo
Offers HQ video content, always worth a check.
Genres: All
Depth: 4
Breadth: 4
File Quality: 8
Accessibility: 6
Database: 5
Search Engine: 5
Writing: NA
Interface: 7
Accuracy: 6
Entertainment: 6
Regions: Global
Media: Video, Audio
Period: 1940 - Now

1.3. Audio Sharing
These services mainly offer 'new' indie music, uploaded by makers and their fans.

Soundcloud
Lets anyone create, record, promote and share music on the web.
Genres: All
Depth: 7
Breadth: 7
File Quality: 9
Accessibility: 8
Database: 7
Search Engine: 7
Writing: NA
Interface: 7
Accuracy: 7
Entertainment: 7
Regions: Global
Media: Audio
Period: 1950 - Now

Mixcloud
Collection of radio shows, podcasts & mixes.
Genres: All
Depth: 5
Breadth: 5
File Quality: 8
Accessibility: 8
Database: 6
Search Engine: 6
Writing: NA
Interface: 7
Accuracy: 8
Entertainment: 6
Regions: Global
Media: Audio
Period: 1910 - Now


1.4. Audio Resources

SugarMegs TIP!
Biggest online collection of live concerts; many files boardmixes or 'taped from the audience'.
Genres: All
Depth: 8
Breadth: 8
File Quality: 7
Accessibility: 8
Database: 7
Search Engine: 5
Writing: NA
Interface: 3
Accuracy: 5
Entertainment: 9
Regions: All
Media: Audio
Period: 1960 - Now

Red Hot Jazz
The Red Hot Jazz Archive is a place to study and enjoy early jazz. It combines the best of books and audio recordings. Don't miss the RHJA forum.
Genres: Jazz
Depth: 7
Breadth: 4
File Quality: 6
Accessibility: 6
Database: 6
Search Engine: 6
Writing: 7
Interface: 4
Accuracy: 8
Entertainment: 7
Regions: USA
Media: Audio, Text
Period: 1890 - 1930

Sinner's Crossroads TIP!
Podcast series on "scratchy vanity 45s, pilfered field recordings, muddy off-the-radio sounds, homemade congregational tapes and vintage commercial gospel throw-downs; a little preachin', a little salvation, a little audio tomfoolery". Also cf. this list of artists played on the program.
Genres: Gospel
Depth: 9
Breadth: 5
File Quality: 8
Accessibility: 5
Database: 5
Search Engine: 5
Writing: NA
Interface: 4
Accuracy: 6
Entertainment: 10
Regions: USA
Media: Audio
Period: 1940 - Now

Awesome Tapes From Africa TIP!
Older and newest music from all regions of Africa.
Genres: World, Pop, Gospel, Rap, Electronic
Depth: 7
Breadth: 5
File Quality: 8
Accessibility: 7
Database: 7
Search Engine: 7
Writing: NA
Interface: 6
Accuracy: 6
Entertainment: 9
Regions: Africa
Media: Audio
Period: 1970 - Now