Voicechanger.io makes it a snap to change your voice to sound like an alien, chipmunk, demon, or many other options. Just upload an audio file or give it permission to access your mic, and you're on your way. It also has a text-to-speech mode for activities requiring additional caution, such as bomb threats, ransom demands, etc.
Note: For some voices (Bane and Dalek, for example), you'll need to put on the correct "accent" for the voice transformation to work properly. Basically, try to make your voice sound like the target voice, and the engine will add some effects to make it sound like the real thing. Another random tip: You'll need to talk relatively slowly if you're using any of the "echo-ey" voices, otherwise the echos will make it hard to understand.
