These are all songs which A) were originally composed by other artists, and B) have been.. re-envisioned... in ways that strike me as really funny.
Baby Got Back - Jonathan Coulton
Holding Out For A Hero - Frou Frou
Ice Ice Baby - Richard Cheese
Iron Man - The Cardigans
Let's Do It - Joan Jett & Paul Westerberg
The Metro - System of a Down
Need You Tonight - Gilli Moon
Ring of Fire - The Texas Gypsies
Rock Me Amadeus - Sturmgeist
Seasons in the Sun - Too Much Joy
Stairway to Heaven - Brave Combo & Tiny Tim
Feel free to leave your own recommendations and suggestions in the comments.