Delay Lama has got to be one of the most fun plugins to use, especially since it is free. It offers both vocal synthesis and a real time animated interface. How awesome is that?
With a little work, you can make it do some really amazing things.
It might not be useful in every-day studio work, but it sure is fun.
To use this plugin in Pro Tools, you need to purchase the VST-RTAS adapter from FXPansion (highly recommended, and worth every single penny).