The greatest gaming achievemet i've ever heard of, which im not sure is even possible. Resident Evil 2, Leon Part A with only using the knife. Honarable mention for Ruby Weapon on FF7.
Considering the 360's system some of the crazier things i've done have been: Saint (Dead Rising) - Evacuate 50 survivors out of the mall, Treadstone Assassin (Bourne Conspiracy) - Beat The Game on Assassin difficulty, One Man Army (GTA 4) - Survive for 5 minutes on 6 star wanted level. I probably have some better ones but I seriously can't place them right now.
Other things I know i've done include beating Super Ghosts and Goblins, Kingdom Hearts on expert without dying, Max Payne on DOA. Also beating X Men and Spiderman vs Arcade on the SNES. Max Stats on all characters on FFX as well. Probably lots more but those tend to stand out.