The 2A03 is an incredibly iconic sound in the world of video game music, but there's one that's just as important yet not as talked about (outside of Europe, at least): the MOS Technology SID. This programmable sound generator was used in the Commodore line of computers, and is essentially responsible for kicking off the Demoscene, an entire sub-cultural movement.
The SID is similar to the NES' 2A03, yet holds its differences. Unlike the 2A03, which has specific waveforms for each channel, the SID presents a bit more flexibility. The programmer has the ability to select which waveform each channel (voice) produces (a total of five), and can modulate the voices together.
Even if you're not familiar with the chip, there's a chance you might have already heard it in some of the music you listen to. The SID chip became popular enough among musicians to warrant Elektron's creation of the SidStation in 1996, a synthesizer based around the SID which continued production for 10 years. Artists such as Zombie Nation and Machinae Supremacy (won't give props to Timbaland due to his blatant plagiarism) have used the SidStation in the production of their music. Zombie Nation's Kernkraft 400 is actually a remix of Stardust from the C64 game Lazy Jones, and Machinae Supremacy uses the SID to embellish their hard rock.
With regard to C64 artists you should check out, definitely anything by Rob Hubbard, Jeroen Tel, and Chris Hülsbeck. There are a plethora of others, but those are definitely the big three. I especially recommend listening to music from Monty on the Run (Rob Hubbard) and the port of R-Type (Jeroen Tel & Ramiro Vaca). See my previous post on how to find this kind of music.
But getting an original SID isn't easy, and the SidStation was incredibly expensive. So, what do you do if you want to create some C64-based music? You turn to the wonderful world of VSTs, of course! There are a metric tonne of VSTs based off of the SID, even commercial products, such as reFX's quadraSID. I can't recommend a good, free C64 VST (due to the sheer number of them), but you can check out this site for a large list.
Sorry if my writing was somewhat dry, today. I was up late hanging out with some friends. Regardless, I hope you enjoyed it, and learned something new!