Jeff Beck uses a ring mod to good effect as a subtle, almost-distortion-but-not-quite-distortion, effect for the lead parts on the track Two Rivers off the Guitar Shop album. That's always been my benchmark for using a ring mod in a non-over-the-top way. Just tastefully different.
Set to extremes, some ring mods produce some interesting, bell-like sounds when fed single notes. This video from Fairfield Circuitry is a good example of bell tones, you hear them right at the beginning of the video. It also runs through some pretty interesting examples of ring modulation on guitar.
Ring mods have also gone hand in hand with synthesizers over the years. The ARP Odyssey comes to mind as a good example where it was meant to work as an almost frequency-controlled distortion on the sound.