Melodyne is pretty crazy. Some of the changes lead to weird results (if you jump to far with vocals), but still pretty cool.

I think the latest version Melodyne is absolutely amazing.

A lot of amateur composing/producing on computers is heavily restricted by a relatively basic set of tools. While the tools may have deeper option sets, hobbyists may take years to discover them. It's rare that a program comes along which allows you to have in infinite amount of sounds to choose from and allows anyone to think of creative ways to recycle existing recordings. When the program is as easy to use as Melodyne, it gives a lot of power to the amateur.