Probably the best bet for creative effects would be Waves Ultrapitch. It's included in many of their bundles and can also be bought separately (they just lowered their prices on everything).
You might also check out Amplitube 3. Even if you don't use the amps, the effects alone are excellent, both stomp and rack versions, including pitch shifters. You can use it on vocals, or anything else.
You might also look at Soundtoys Native Effects. This company was founded by guys who used to work for Eventide and they have some unique stuff.