The Simon Laven Page
Shampage, a brilliant Chatterbot program whose language base is totally configurable. You can set it up anyway you want it, and if you set it up well enough it can become a truly amazing program.
Eliza, the virtual psychoanalyst. Originally created by MIT scientist Joseph Weizembaum. Several versions available here along with backround information on the worlds most famous chatterbot.
Fred, the Functional Response Emulation Device. The first program in an ongoing experiment to explore Natural Language communications between people and computer programs.
Claude, when the author released this program to the world the 'readme.doc' said: "Claude isn't very smart compared to you". This is true, but he's also a clone of the classic Racter Chatterbot.