Here are some freewares with which you can modify your voice without any special equipment, but a microphone.
Introducing Morphvox Junior, a free voice changer software that can modify your voice if you don’t really like your own voice in order to improve your voice quality. You can also transform your own voice into the voice of a child voice, an adult voice, an old people voice or even a robot voice to fool your friends.
For example, you can make a prank calling to friends via Skype, instant messaging and Voice over Internet Protocol (VoIP). Have fun!