Calculating the color palette of the VIC II
This is a very in-depth attempt at faithfully reproducing the C64's colors using a single calculation model. The author, Philip "pepto" Timmermann, is widely known for his pepto palette which was the predecessor to his new colodore palette. All C64 graphics on this website use the colodore palette, unless noted otherwise.

High Voltage SID Collection
Quote from the website: "HVSC is the ultimate SID tune collection featuring over 41,200 popular and requested SIDs from the Commodore 64. The collection includes SIDs (aka C64 music) from games, demos, intros, etc. HVSC has been in the making for fifteen years and is the result of many contributors."

Gamebase 64
A massive database with the goal of collecting information about all C64 game software. This site has proven to be a very useful online reference when I'm doing research on old C64 titles.

Lemon 64
Another website with a large list of information about C64 games, including links to scans of game covers and magazine reviews.

The C64 Scene Database
This is a great resource to find C64 scene-related information, past and present.