Some time ago a C64 pixel art gallery emerged: C64pixels.com. The gallery is constantly being updated with old and new C64 graphics and already contains pictures to amaze for a good while. And there is even more content available if you register to the site.
The gallery is executed really well with emulation for actual display picture. The pictures have filters applied to them to simulate for example CRT-monitor scanlines, the softness of old monitor or TV and the colour bleeding. Also the interlaced pictures are flickering in the gallery! The result is close to how the pieces actually look on authentic hardware.
Some of the artwork is magnificent just as they are, but to really see the beauty in the pictures one has to understand the limitations that the artist have to work within. The fixed palette of 16 colours and the relatively low resolutions are just the beginning. Here is a short and nice introduction to different C64 graphics modes, their properties and limitations. Here is another, more technical approach to C64 graphic capabilities. Have a look into these documents, and then to the best pieces in the gallery!