Pitfall II was the first cart I remember having a coprocessor -- the DPC, only used for this game since the 2600 was on the way out: http://atariage.com/forums/topic/144075-how-exactly-does-the...

NES carts actually have an audio pass-through that can replace/remix the built-in APU sound. Atari 2600 does not, but the DPC gave it three-voice music. How? The CPU has to read from the DPC once per scanline, then manually shove that value into the volume register, generating the audio waveform at a 15.7 kHz sample rate.