These could be games you beat, milestones in MMORPG's, or actual "Achievements" in Xbox 360 or awards on PS3. Heres some of mine:
1. Beating Ninja Gaiden II on Path of the Master Ninja. I can blow through this game in 3 hours normally. This took me about a month and a half to do. Ironically the last boss was the easiest on this setting. The gamer pic I got for beating this will never leave my profile pic.
2. Hitting Level 70 in Final Fantasy XI online. The hardest level grinding MMORPG ever. I leveled my Elf Paladin and promptly quit the game (and MMORPG's) forever. They kill real friendships!
3. (Almost) Winning a Marvel vs Capcom 2 tournament in Vegas. I used to be hardcore into arcade fighting games. I made it past the regional tournament here in 2003 and got an invite to play at EVO in Vegas. I had to drive myself and pay for a hotel, but it was cheap at the time! I made it pretty far until I barely lost a semi-final match. My team was always Ryu, Cable, and Magneto. Can't wait til this hits XBLA.
4. Beating Final Fantasy II (SNES). This was my gateway drug to the Final Fantasy world. I played the one on NES but never finished it (I liked Dragon Quest better), but this game was amazing. From the mist dragon to getting the Excalibur sword in the underworld, one of the best true RPG's ever. I tracked down the Japanese sequels after this for years until they were re-released on Playstation.
5. Beating Mortal Kombat before anyone in my town! I grew up north of Denver in a boring town called Commerce City. Our closest "arcade" was in a little 8 lane bowling alley in a part of town known as Derby that had one or two arcade games. Somehow, the owner got Mortal Kombat around the time it was released everywhere and about 20 of us kids lined up the morning he got it. While 3 people bowled, we all took turns. After the crowd died down several hours later, we all took a shot at beating the game. I learned sub zeros freeze move and just kept using it to blast my way to Goro. It took a few tries to beat him, then Shank Tsung was easy. I was like the hero of arcade kids for a month!
Thats it for now, I'll think of more later I'm sure.