diskimage.c – Library used in QtCBM to access D64 image content, by Per Olofsson
Commodore Free – A Modern Magazine dedicated to Commodore machines.
Carlsen Electronics– Very knowledgeable in the repair of all things Commodore.
C64.com – Fantastic place for demos and D64 disk images.  Interviews and more…
Lemon64 – An excellent review site with screenshots and ratings.
Retro Inovations – Home of JiffyDOS. An absolute must for speeding access to the 1541.
Bombjack – One of the absolute best places to find great reference books on Commodore machines.
GameBase64 – Another great place for disk images with searchable game database.
C64Copy Central – In-depth page on making the cables used for transferring images with QtCBM
OpenCBM – This is the open source software that forms the basis of QtCBM.
DeepSID = A giant library online of SID music classics from games/demos. Built-in player also!