MorphVOX Junior allows you to change your voice in real time. It can be used for online gaming and instant messaging (Skype...).
Audacity is an open source (free) software which allows you manipulate audio files. It allows you to apply several effects and filters to your voice: echo, phase, etc.
Clownfish is an application offering many features (spell check, translation, text-to-speech,...) for Skype, including an option to change your voice.
The paid version of MorphVOX (demo for 7 days) is enriched with hundreds of voices and special effects (distortion, background noise). You can change your voice with filters. Perfect for online gaming and suitable for all VoIP software (Skype, Google Talk, Yahoo Messenger, MSN Messenger).
AV Voice Changer is a software that allows you to modify your voice in real time, with an wide range of effects.