Many I've seen suck, but recently I've stumbled across a couple of high quality ones. 
Here's a good one!  Show your faves!  Discuss!

https://www.youtube.com/watch?feature=player_embedded&v=W3dQTWmjtMs

Early Mashups ?    Brenton Wood - Psychotic Reaction (With the backing track from the original , by Wood's labelmates , The Count Five.)

Shadows of Knight - Gloria '69 ( featuring snippets from the original SOK version , plus a new vocal and instrumental track. It does'nt work.).

THERE WAS AN ELVIS BOOTLEG I HEARD , ONCE , THAT CLAIMED TO BE DUETS WITH ALL THESE FAMOUS SINGERS , THE BEATLES , TOM JONES , JOHNNY HALLYDAY , YOU NAME 'EM. THEY MERELY PLAYED TWO RECORDS TOGETHER, USUALLY AT DIFFERENT SPEEDS , TO MAKE THEM APPEAR TO BE SYNCHRONIZED. IT WAS A FUCKING MESS !

Here ya go:

John Battles said:

 I still like the first one I ever heard , the Stooges/Salt 'n'pepa mash- up.
"ooh , baby , baby , oo, baby , baby - No fun , mah babe , no Fun. DA NA DA DA NA NA NA DA DA DA DAAAA NO FUN , MY BABE , NO FUN - BABY BAY , OOH ! BABY BABY !

Long before this mashup hooey , Rhino released a record by "The Seven Stooges" . 3 Stooges impersonators (Naturally , no Shemp , the uber- Stooge.) doing "I Wanna Be Your Dog".

Yeah, John's right, our very won Spite Classic does the best mashups!

this guy has some amazing stuff, goes by the name ccc 

he does unbelievable things with 60s tunes, and has seriously pissed off the beatle estate.

have a dig around 

http://www.youtube.com/watch?v=e9VjFSB2PX0

and his blog 

http://www.mashups.blogspot.co.uk/

Hahaha!

 I still like the first one I ever heard , the Stooges/Salt 'n'pepa mash- up.
"ooh , baby , baby , oo, baby , baby - No fun , mah babe , no Fun. DA NA DA DA NA NA NA DA DA DA DAAAA NO FUN , MY BABE , NO FUN - BABY BAY , OOH ! BABY BABY !