Here is the full list of all 43 Over 9000 Zombies! achievements.
The first day is always the hardest, except in this case, it's the easiest.
Kill 1000+ Zombies!
Kill 9,000+ Zombies!
Kill 90,000+ Zombies!
Kill 900,000+ Zombies!
Vote for Zombie Thomas Jefferson, or don't.
Making some progress...
You won that race.
Green IS good.
You watched the world burn, and survived. It was kind of cool.
Lots of progress!
Not a wimp.
It didn't taste like chocolate ice cream either...
You didn't donate your organs...
Your world didn't end!
Don't call me slow!
Decreased Zombotropy.
Quoth the Raven "Nevermore."
Blood and Flesh can be trusted.
A jail is an isolated system.
If only he had the body of a horse...
Terror possessed me then!
Health is the greatest gift, a close second is BRAINS!
Abused patience turns to fury!
The zombie world is independent of our will.
Construct a turret from scrap metal!
Construct 100 automated turrets.
Construct 1000 Turrets from random scraps of metal!
Get 10 kills per second!
Get 30 kills per second!
Get 5,000 damage per second!
Get 15,000 damage per second!
Collect 100 Power Ups!
Participate in a multiplayer game.
Kill over 1000 crabs!
Kill over 1000 zombie dogs!
Kill over 1000 zombie crows!
Kill over 1000 zombie eyes!
No 8th symphony for zombie Sibelius!
Derail the Waltz of the Undead!
Voted the greatest zombie of all time!
The army of the undead will march at daybreak!
Zombie Napoleon meets his zombie Waterloo!