The SID chip itself is digital, but the filters that work with it are of the analogue type. Thus, the C64 is a hybrid digital/analogue synthesizer. I don't think the oscilators are analogue, but the filters surely are.
If you have a C64 around and spare time, make a cable that will allow you to use the video port's AUDIO IN pin. Connect a guitar to it. Makea little program that would tweak teh filters. Now you can make wah-wah effects and other stuff on the guitar, using the C64s analogue filters! fab eh?
Some C64 freak friend of mine told me about this, he always uses this, and with a paddle controller, is like having a pedal