Mine was:
Double Dragon  - Bare Hands (20G)

I got it in NHL 2008 - don't remember what for.

AN-I-MAL! achievement on Rock Band. It's what you get for beating expert drums.

I got the one for hard/medium, and easy drums at the same time, but the expert one is the only one that matters. ;)

Top Artist - Rock Band

Right Hand of God
Complete GH3 on Expert

Raining Blood was a bitch but I somehow managed it

Mines was completing the second mission of Battlefield Bad Company on normal.

Just won 5 tug-o-war matches in Rockband Online

COD4 - Mile High Club

COD 2 - Veteran of El Amein (60g)

Don't know what it's called, but it was for Topping the Race Driver: GRID leaderboards.

Civ Rev - We the people (15g)

Something on UT3, I dont remember the name.

"Damned if you don't

Kayne and Lynch 30g

I no crap game but i need to finish all my games lol
"

Mongoose mowdown-5g

PuzzleQuest
Renowned Hero - 10g

Complete the tutorial in Guitar Hero: Aerosmith - 10gs

Playing as a elf in 100 games on shadowrun-10g

Assassin's Greed
You completed all 9 assassin missions.

20

Complete Hardcore Playthrough - 25G in Mass Effect.

Unreal Tournament 3:

Winning is the only thing
Finished with the highest points in a Ranked match.
10   Acquired 20/7/2008

On Top of the World in Battlefield: Bad Company (20G).

Justin

shoot the goldfish (alone int he dark) 10G

Not an achievement but i got a trophy for doing a flatspin 360.

the TV collection achievement on COD4, last one :D
Next is Top Spin 3 and Aerosmith

on July 12, Scholar achievement for Mass Effect.

Battlefield: Bad Company

The Hypochondriac

Dial B for Bomb-GTA IV

10 GS

Zen and the Art of Reloading Part 2

"Zen and the Art of Reloading Part 2
"

Completed Disc 3 in Lost Odyssey

Multi-Monster Encounter  (5 G)  It's from blue dragon, I got it a few days ago.

My name is Sam-25G
Rainbow six Vegas 2

Flint Marko - Spider-Man 3..... lol got really bored one day and just rented it... but haven't played with my 360 since MGS4 came out.

Specialist 10 - For Battlefield Bad Company

The 100 pt one for beating Assassin's Creed

Guitar Hero 3
Your First Big Score - Score 250,000 points in a single song (5G)

Hoarder(5G) from Doom.

AN-I-MAL on Rock Band

For beating Expert Drums

Ninja Gaiden II - Falcon's Talons Master (5G) - Complete the game using only Falcon's Talons.

I think it was for diveing out of a helecopter and falling for 3 seconds.

something in skate, i know i suck..

I think it was completing a full race without getting time taken away on Forza Motorsport 2

I'm Special! NCAA 09