Feel free to post your own if you have any, I personally don't, so I'll post some i remember from the web:

SF

SMB3

Share some videos with crazy skills you know!

Paging Dr. SHMUPGurus!

That Street Fighter video is crazy...

I remember one with an asian dude playing DDR on both steps and nailing everything while playing with both hands and feet... can't seem to find it though

Wow, everytime I see a Super Mario Bros. 3 run through, I see new stuff with the game I've never seen before. Anyways, isn't that one done with an emulator slowing it down so much?

The SF video has been around a while, and that was beyond the EPIC proportions and well into legendary.

Mortal Kombat, Street Dancer vs small kid?
http://www.youtube.com/watch?v=UdjFQ4UK19Q

HKGolden in Re4

Mango combo in Pound 3

A couple from the top of my head...

Speedruns of Half-Life in half an hour and all of Quake in 12 minutes is pretty damn amazing to watch! Vids are easy to find.

Edit; that RE4 one, that must be some of the shittiest AI I have ever seen in a game.

Yah that Daigo vs Justin one is way old.

Here's one I find cool it's the 2008 Evo highlights the most amazing one is My Dirty is Better >

Also this one is awesome as well! Daigo vs. Alex Valle the only American to beat the Japanese legend on American soil.

That SMB3 video is a fake. If you play it at the same time as you play the game, he's running way faster than is actually possible in the game. I've tried it. He slowed it down to cheat his way through it, and when he sped it back up he sped it up a little too much. The guy who made the video admits this all too, but it's in Japanese, so it still pops up on the internet every few years with people thinking it's a legitimate speedrun. It's still amazing to watch, but it must be noted that it's in the "superhuman impossible cheating" category.