Melissa is a
virtual assistant for OS X, Windows and Linux systems. She currently uses Google Chrome's speech-to-text engine, OS X's
say command, Linux's
espeak command or Ivona TTS along with some magical scripting which makes her alive, developed by Tanay Pant and a group of sorcerers. The Web UI for Melissa has been designed by Nakul Saxena.
Please refer this book for a complete introduction and explanation of the dynamics (and concepts) of Melissa.