Salt Boy – Complete the Salt Factory without dying.

Brimstone Boy – Complete Hell without dying.

Maggot Boy – Complete the Rapture without dying.

Dead Boy – Complete The End without dying.

Nostalgia – Unlock a single retro warp zone.

Living In the Past – Complete 5 retro warp zones.

Old School – Complete 10 retro warp zones.

Retro Rampage – Complete all retro warp zones.

The End – Beat the light world.

The Real End – Beat the dark world.

Golden God – 100% the game.

I Have Crabs! – Unlock the Head Crab (10 bandages).

Metal Head – Unlock the Machinarium Robot (30 bandages).

I Smell something Fishy… – Unlock Naija (50 bandages).

MS PAINT RULZ! – Unlock RunMan (70 bandages).

Vx6 – Unlock Captain Viridian (90 bandages).

Accidental Arsonist – Unlock Mr. Minecraft (100 bandages).

The Commander – Unlock Commander Video.

The Bootlicker – Unlock Jill.

The Jump Man – Unlock Ogmo.

The Fly Guy – Unlock Flywrench.

The Kid – Unlock the Kid.

There's also one achievement apiece for beating the secret levels, -1 through -6.