Where there's smoke, they pinch back.

Yeah a lot of these just use clips from the same exact sources and it gets old kind of quick, mostly because, as Syd said, it's the same exact clips in pretty much each one, though some of them are kind of original in their usage. Every now and then you'll get one that throws in something unique, but it's rare.

If you think you can handle the sheer horror, there's an entire site dedicated to these things here, complete with it's own Wiki.

I like how Murdar's Poop was removed by angry Fox executives.

Basically it's what happens when you get bored and decide to fuck around in Windows Movie Maker. It's essentially just a mash-up of random, unrelated clips thrown together from a variety of sources (usually the Super Mario World and Sonic cartoons, and clips from the shitty Zelda and Mario Philips CD-i games, other sources are often used though) made into a spectacular mess. They're often overdubbed, sped up, slowed down, flipped/rotated, given seizure-inducing colors, etc. Apparently it falls into the "so random it's funny" category.

I think some of them are hilarious, while others are meh. The first one Syd posted is pretty funny.