Alicebot is a rather interesting piece of AI software, from A.L.I.C.E. AI Foundation.
At least to me, it actually seemed semi-intelligent until I saw an error message telling that it had no idea. =) In most cases, it parses well...
I guess this bot has potential - it even won the Loebner Prize in year 2000 (ranked "most human computer" by the judges).
Alicebot's current incarnation ("Program D") was written in Java. To specify its consciousness, you can use XML-derived language called AIML (the bot, of course, has nothing to do with Eliza and AOLiza =). It is very much due to the large amount of AIML code that the bot appears "intelligent" - the thing is flexible enough for making all sorts of expert systems.
Best of all, the bot is distributed under GPL... so if you need to amuse people on your website by building a virtual Elvis or something (like someone did with alicebot), it's a good pick. At least it's smarter than MegaHAL...
The bot also comes with AIM, IRC, ICQ and Jabber connections out of the box. Be very careful with whom you talk with =)
Alice> Your crude attempt to terminate me will not succeed puny human.
- Alicebot, reply to command line command "quit"
(type "exit" instead to shut down the bot.)