Eliza's not new, of course, but it's fun to play around with. (It was first made in the 1960s, and it's probably been ported to every single computer ever)
I had an alternate version on the Mac SE called Azile ("Eliza" spelled backwards). Same basic triggers and responses, but twisted into insults. (You can still actually buy this one directly from the programmers, but good luck getting a modern computer to run it)