1. Beating Horde wave 50 on Gears 2. Hardcore difficulty with just myself and one friend.
2. On Halo 3 getting the splatter and flag kill medals with one kill (On Avalanche I flew through the man cannon into the enemy base in a warthog, got out, grabbed the flag, and while I had the flag my 'hog flew back through the cannon and crushed a guy).
3. On Oblivion getting a character that was leader of the Fighters Guild, Mages Guild, Dark Brotherhood, and Thieves Guild. As well as having completed the main quest and the arena. And beating pretty much every side quest and owning every house in the game. So yeah I pretty much did everything there was to do.
4. On Fallout 3 actually being able to discover the entrance to Vault 87. If you've ever been there it's a pain in the ass to even get close enough to discover it.
5. Also on Fallout 3, killing a Super Mutant Behemoth with a very damaged Sledge Hammer.
I had one other but I can't for the life of me remember it. =P