It may have escaped your narrow, short sighted attention that games developers have been putting secret achievements into their XBOX 360 games, which have very exceptional parameters to meet. These don't necessarily show up in your achievements list, but you can unlock them all the same. Many games actually have an extra 500 points associated with them. Here's a list of some of the currently known ones:
Gears Of War 2
Form Of Flattery - 50 Points: Successfully impersonate Cole Train over XBOX Live by shouting "Wooo baby!" and "Lookit all dat juice!" at downed opponents. Constantly. Forever.
WTF - 100 Points: Score a birdie in the hidden Locust Darts mini game using a set of dominoes to topple the house of cards.
Best Offence - 100 Points: Seduce Anya in the secret bunker level back at Jacinto.
Queen Slugforabutt - 250 Points: Seduce the locust Queen and have Anya walk in on you.
Whoopsie Daisy - 500 Points: Pop this in the wrong console, mistaking it for Prototype.
Guitar Hero World Tour
Rock Superstar - 200 Points: Complete a choreographed routine in your front room with the guitar involving 25 star jumps, sliding on your knees, playing behind your back, and jumping off of at least 2 pieces of furniture.
Bass To The Place - 50 Points: Convince a friend to play Bass. No one wants to play Bass.
Animal - 250 Points: Play the drums for an hour without having them break on you and needing to be sent back.
Left 4 Dead
Karaoke Time - 200 Points: Hum The Specials - Ghost Town on repeat whilst playing.
Karaoke Time 2 - 200 Points: Hum Thriller on repeat whilst playing.
You'd Get Shot In The Army... - 100 Points: Run away at the start of the game and leave all of your team mates to die.
No Laughing Matter - 150 Points: Don't laugh at the stupid plot twist involving mind control devices right near the start of the game.
Spare Any Change? - 150 Points: Press pennies into a tramps eye whilst shouting "Here's your pissing change, mate!".
Entrepreneur - 200 Points: Successfully found an underground hobo boxing syndicate.
Waster - 50 Points: Stay in the apartment for six hours real time, watching TV and smoking weed. Just like your real life, you pathetic waste of space.
Realism At It's Best - 50 Points: Flip a bus over by driving off of a highway, landing on your roof, and watch as the bus miraculously rights itself and slides back onto it's wheels.
In Game Environmentalist - 50 Points: Complete the game whilst only driving hybrid cars and without farting.
Not Now Cahsin I'm Busy - 50 Points: Throw your mobile phone into the sea.
Bombs Over Baghdad - 100 Points: Hijack a plane and fly all the way across the ocean to see your mother.
The Latest Model - 100 Points: Upgrade your mobile to an iPhone, then download the app which guesses your penis size and uploads the results to Blogger.
Brucie Bonus - 100 Points: Find Brucies steroids and take them until you turn into The Hulk.
Devil May Cry 4
Halfway There - 250 Points: Realise that you completed the game two hours in, and now they're just making you replay the whole game a second time, even down to the boss fights.
We Need A Pool Table - 250 Points: Beat the old priest, and send him to a retirement home.