Siri9000 is a useless but fun jailbreak tweak that transforms Siri into HAL 9000 from the film 2001: A Space Odyssey. The Hal icon is easily installed by downloading the tweak from the Big Boss repo, and activating it from WinterBoard. Getting the Hal 9000 sound effects to work takes a little bit more effort.
I should warn you that the tweak does not convert Siri's voice into Hal's. It only changes the Siri start up sound to a few Hal quotes from the movie ("Good evening Dave", "I'm sorry Dave, I'm afraid I can't do that"). To activate the Hal 9000 sounds you will have to know how to transfer files on your iPhone with SSH and have OpenSSH installed.