A work of many hours of research and thousands of re-recordings blowing up into a short movie of ludicrous speed and glitching. I just love watching some of those.
http://tasvideos.org/
Check out for example:
Brain Age image recognition broken in half: http://tasvideos.org/1734M.html

Super Mario 64, 0 stars, 5 minutes, ass bouncing and "So long, gay Bowser!": http://tasvideos.org/2016M.html
Or a completionist run http://tasvideos.org/2208M.html

Goldeneye (N64) - a tale of casual headshots and wall-rubbing: http://tasvideos.org/1750M.html

Tetrisphere's time attack mode forgets what the game is about. Also, insane hypnotic speed. http://tasvideos.org/530M.html

Richter Belmont killing Dracula (again) in about ten minutes in Symphony of the Night: http://tasvideos.org/1275M.html

Naked Nikujin ninjas neutralized (in 2 minutes): http://tasvideos.org/2038M.html
Regarding the last video - yup, there is a program now allowing the use of savestates and other tools in a few Windows games. They call it Hourglass

If you like speedruns, check out Awesome Games Done Quick on youtube. I've seriously wasted hours just watching dudes speed run games.

don't forget these guys > speeddemosarchive.com

What is a tool-assisted speedrun?

A speed run of the game recoded using savestates, slow frame-by-frame recording, luck manipulation, code oddities, glitches.

I like speedrun (sometimes I do one, like for Ico or Silent Hill), but those tool-assisted I think are useless...it's like cheated speedruns if you "cheat".

I'm guessing that there is some small advantage to looking at the floor the entire time? Either makes you slightly faster or harder to hit? It results in a dull video.

edit: Reminds me of some speedruns where the player kept equipping a grenade, I think. It made him run faster. Maybe it was RE4?

The point of it is to see how far you can push the limits of a game and use its own flaws to your advantage; of course it's not "fair" but that's exactly the point. How many times has the computer cheated against you? Well, now the shoe's on the other foot.

Take that you cheating bastards!!

edit:Damn apostrophe got away from me :P

Sweet revenge. They have a speedrun of the original King's Bounty, where the buried treasure spawns right under the starting location. Total gameplay time is under ten seconds.

I especially like the slightly more unusual videos.

Family Feud isn't the kind of game you'd think was very TASable. And it isn't - but you can still get a lot of humor out of the broken parser: http://tasvideos.org/1248M.html

Pokémon Yellow is breakable beyond belief: you can beat the game in a matter of minutes, and in multiple ways, too!One option is horrible glitching and save corruption, but you can even go so far as to reprogram the game (which is also done in this video, although there the goal is not completing the game).

There's also 4 Mega Man game with one set of inputs.

And finally, the shortest 5-player game of Monopoly in the history of the world.

This one seems like it could be the basis for a game in itself.