Modify the voice quality to make it softer or more metallic.
Maintain constant voicing (eg with those touch-the-neck-when-speaking doodads).
Modify the pitch as you speak (singing). Talk in a very high or low voice (but some vowels are harder to pick when singing very high).
Modify tempo while speaking, eg drag out a single syllable for a long time (Dave Hughes), or double or halve tempo in the middle of sentence.
Remove pauses. Add pauses. Add "ers" and "ums".
Pinch your nose shut.
Modify the volume of a recording.
Modify the speed of a recording considerably (Alvin and the Chipmunks) (note that the physical correlate of this would not be a change of pitch, but scaling of the entire vocal tract).
Add considerable background noise.
Add considerable echo and/or resonance (eg in a room with reflective walls).
Whiten a recording using short time LPC, shuffle whitened samples within each frame, unwhiten (sounds whispered).
Knock out large chunks of the frequency spectrum. No part of the spectrum is essential.