I just added a vocoder feature to the latest firmware for my Patch Submodule-based xVox Harmonic Pitch Shifter module.
It does robot voices as you would expect but things get really interesting when you use two different oscillators for the carrier and modulator. The resultant cross-modulated audio signal feeds the pitch-shifter engine which does chords and chord progressions. Modulating both oscillators creates shifting patterns of harmonics, and poly-rhythms when the oscillators are enveloped or triggered.
There is more information about my module on my website: