This is a great site, this page has a bunch of MP3's of the most horrible, wacky and funny audio ever recorded, most of it was at one time called "music." Bad renditions of songs, along the lines of William Shatner's Lucy in the Sky with Diamonds, dominate but there are also TV show outtakes, "original" songs, and various kinds of spoken word pieces.
Soundgarden's Black Hole Sun by Steve & Eydie
Leonard Nimoy "singing" Put a Little Love in Your Heart
Johnny Cash singing I Walk the Line in German. There's a whole "in German" section.
You might have heard of "mash ups," where someone strips the vocal track from one song and replaces it with the vocals of another song. I've only heard a couple and didn't think too much of them until now. This is Eminem lyrics over some classic ragtime music. I almost wet myself laughing.
Eminem vs. Ragtime (Uncensored) by Freelance Hairdresser
Eminem With or Without Me has a whole collection of Eminem mash-ups that based on their descriptions I must listen to very soon.