SidFind
c64.org's SID music search engine.
http://www.c64.org/sidfind/

Composers.c64.org
A collection of pictures of C64 composers, samples on how to pronounce their names and their best SID tune.
http://www.composers.c64.org/

SIDManager
SID file and playlist manager. Designed to be used with the High Voltage SIDs Collection.
http://members.lycos.co.uk/SIDManager/

Skytopia - Greatest C64 Music
The top Commodore 64 games music classed according to melodic content, intricacy and general atmosphere. Each C64 tune is dissected, reviewed and available for download.
http://www.skytopia.com/games/c64/c64.html

Sidmuzik
Enhanced stereo recordings of SID tunes (Ogg Vorbis format).
http://sidmuzik.brianstagg.co.uk/

HVSC Scanner
Windows tool that scans a directory tree for SID files and dumps information into a text file.
http://www.informatik.uni-oldenburg.de/~flohinz/hvsc/

High Voltage SID Collection (HVSC)
Most complete SID tune collection, containing over 35,000 SID files.
http://hvsc.c64.org/

prophet64
Synth software for the C64, resembling the handling and sound of the Roland 303/606/808/909 synthesizers. MIDI compatible.
http://www.prophet64.com/

C64 Music
Journal about Commodore 64 music in the real world and related stories.
http://c64music.blogspot.com/

High Voltage Music Engine Collection (HVMEC)
A downloadable archive of all known Commodore 64 software used to create music.
http://digilander.libero.it/ice00/tsid/hvmec.html

C64 Take-away podcast
A podcast featuring SID remixes, original SID music, and news.
http://c64takeaway.com/

Compute's Gazette Sid Collection (CGSC)
Huge collection of C64 music in the popular SidPlayer format.
http://www.btinternet.com/~pweighill/music/

SID.OTH4.COM
SID music in mp3 format, recorded from a HardSID card.
http://sid.oth4.com/

The C64 Music Webring
The webring for C64 music related sites, especially the SID and MOD formats.
http://www.webring.com/hub?sid=&ring=c64music&id=&list