This is nothing new - it made the rounds on the internet last year - but just in case anyone reading hasn't seen them, I highly recommend the St Sanders "Shreds" videos. Hy-larious.
For the unaware, these are by a Finnish video artist named Santeri Ojala. What he's done is take video performance clips of various guitar heroes (Van Halen, Clapton, etc.) and dub new guitar solos over the videos. Bad guitar solos. Rather like the "experimental free-form jazz odyssey" Spinal Tap is forced to perform after Nigel quits.
Think of it as sort of like an instrumental music version of a What's Up, Tiger Lily?/J-Men Forever kind of thing. The thing is, he does such a good job simultaneously matching what the video shows them playing (well enough to actually fool some people), but at the same time being so completely out of character... well, really, it's indescribable. Just watch. I particularly recommend the one featuring Ozzy Osbourne on hand-claps.
Steve Vai apparently has a sense of humor*, and loved them. Yngwie Malmsteen, not so much: Rumor has it that he's the one who complained to YouTube about copyright violation**, resulting in the creator having his account banned. Oh well, the videos are still there, so I consider it my civic duty to ensure as many people see them as possible...
Also: The non-music related one is fun, too.
*Probably a necessary survival trait in Frank Zappa's band, where you run the risk of having your groupie exploits immortalized in a song like "Stevie's Spanking".
**Despite this obviously being parody, and therefore falling under the Fair Use exception.