Speaking of RE4, there's some amazing playthrough of it using only the knife (in a few spots it's impossible, in which cases he uses the starting handgun with no upgrades). I think it's still incomplete, but he got as far as chapter 5-2 before stopping. Youtube channel name is "KaiWii"
Also there's this freaking incredible
"no equipment" playthrough of MGS3 where the player uses absolutely no items whatsoever, no camouflage, food, no anything and still gets an extremely quick FOXHOUND ranking on European Extreme. Fun watch.
This is a DMC3 "NO GROWTH" playthrough, where the player Heretic plays through the entire game on the hardest difficulty, Dante Must Die, without upgrading Dante or any of his weapons at all (one of his styles levels up once, but that's unavoidable). Very, very, very difficult:
Oh yeah, and there are people that play through Ninja Gaiden Black on the hardest difficulty using these absolutely crazy strategies to max out their karma scores, which requires beating all of the in-game battle timers and doing all sorts of other sometimes wacky and often brutally challenging things. I could barely scrape through Master Ninja mode at all, but there are people that just make the game their bitch. Totally insane people.
As for me, I once beat MGS3 on Extreme with purely CQC (but camo changes and the cardboard box and stuff allowed) using a strategy I saw on Youtube. I got a FOXHOUND ranking by beating the game in under 3 hours, the requirement being 5, and I still have the save file in case I ever need to brag about a game that's been obsolete (though still awesome) for 5+ years
EDIT: shit i just got ninja'd at least twice, that's how long this post took me