Shatner covers Pulp's "Common People."

Shatner sings "Lucy in the Sky with Diamonds."

Shatner sings "Mr. Tambourine Man."

And, of course, the infamous, the classic, Family Guy-parodied, Shatner sings "Rocket Man."

But if you think all that's scary, then mister, you haven't seen Leonard Nimoy sing "The Ballad of Bilbo Baggins."

UPDATE: Shatner sings Neil Diamond's "Shiloh."

Brent Spiner (Data) singing "Time After Time."

UPDATE 2: Shatner reads from Henry V. What a world.

UPDATE 3: I can't link directly to the MP3s, but you MUST go to this site and listen to a) Leonard Nimoy sing "Put A Little Love In Your Heart" but b), and especially, Jerry Springer singing "Mr. Tambourine Man." Trust me, listen to Springer. It will change your life. Other good stuff there too.

UPDATE 4: I found the clip of Stewie doing Shatner doing "Rocketman." For some Unfortunately, the people who put it up synched it up with the real "Rocketman," but this is the best I can do right now. New link.