Oh wow, how did I pass so much time online without hearing this? It's an MP3 of a recording session by William Shatner in which he's recording a voiceover of some sort. But when the producer guy decides to start making specific requests regarding Shatner's delivery of said voiceover, he unwittingly unleashes Shatner's Inner Jerk, with utterly hysterical results. This was played on the Howard Stern show, so after the Shatner stuff is over, there's some salty commentary by Stern and cohorts. But wow, that laugh of Shatner's...you can just hear him thinking, "OK, jerk-off producer guy, you want to mess with Teh Shat?"
(Found in this MeFi thread)