3D Fluff: Created by Matthew O’Neill and Janine Paulke. They provide a number of free tutorial videos through their youtube channel, and an eBook on Product Design & Rendering.
3D Total: A number of project breakdowns of finished renders. There seems to be an emphasis on cars, tech, and creatures.
3D World: A youtube channel with a number of tutorial videos. Not exclusive to C4D.
Arab 4D: Appears to be an online Cinema 4D forum for those that speak Arabic.
Base80: A grey/orange colors scheme with a strong focus on the more technical aspects of Cinema 4D. Xpresso, dynamics, etc.
Beeple: Beautiful stills and animations by Beeple. Remarkably he posts many of his scene files for free - feel free to tear them apart and figure out how they work.
Best of Cinema 4D Tutorials: Sort of like an RSS feed, this channel is regularly updated with the best c4d tutorials found on vimeo. The content seems to skew towards mograph.
BuzzBox: Architectural rendering, with a focus on VRay. They have two tutorials on architectural exteriors up right now.
C4D Avenue: German page for the Cinema 4d users in Zürich.
C4D Cafe: One of the more active online Cinema 4D communities. Emphasis seems to be on beginner topics. They also post a number of in-depth reviews of the latest Cinema 4D and plugin releases.
Greyscale Gorilla: Greyscale Gorilla is a Cinema 4D Tutorials blog primarily hosted by Nick Campbell and Chris Schmidt. You’ll find a number of tutorials including a complete introduction to Cinema 4D. The site largely emphasises motion graphics tutorials, with a little bit of VFX, and general tips and tricks.