MegaHAL is an AI conversation simulator, originally written by Jason Hutchens for the 1998 Loebner AI Contest, that allows users to talk to a psychotic AI. Unlike traditional conversation simulators (see Eliza), MegaHAL learns from what users tell it, and can build up a vocabulary on its own. MegaHAL can also learn other languages, and it can speak to you with any voice you have installed on your Mac. MegaHAL's dialogue can invoke a whole range of emotions; you can never predict what it's going to tell you!
This is the same version of MegaHAL that comes with Debian Linux. It's been Carbonized to run under Mac OS X. MegaHAL is distributed under the GNU Public License; source code is included.