Besides the obvious like pitch shift, I have a few favorites such as Prosoniq's Morph and PiWarp, iZotope Spectron, and GRM tools as well as some of the Pluggo suite. What are your favorite plug-ins for use in building creature vocals?
(Edited because I feel like I gave the wrong impression with the initial question -- I'm not saying "please tell me which plug-in will magically create great creature sounds," but rather "I'm curious as to which plug-ins you've had good results with, and for which specific aspects of the vocals you were creating." Obviously, creature creation is a lot more complicated (and fun) than passing a recording through some wondrous RydstromVox Plus plug-in. :) Sorry if it sounded like that's what I was asking.)