You can unlock cheats by finding the required number of Intel Laptops that are hidden throughout the single-player campaign.

2 - COD Noir (adds a black-and-white filter)
4 - Photo Negative (inverts color palette)
6 - Super Contrast (greatly increased color contrast)
8 - Ragtime Warfare (adds black-and-white filter and piano soundtrack)
10 - Cluster Bombs (grenades explode into a pile of grenades that also then explode)
15 - A Bad Year (enemies explode into tires once killed)
20 - Slow Motion (tap the melee button to initiate "bullet time" effect)
30 - Infinite Ammo (never run out of bullets)
30 - Zakhaev's Sons (all enemies are Zakhaev, with a rocket launcher)
30 - Explosive Lemons (grenades are lemons)
30 - Melon Head (enemies' heads are watermelons that burst apart when shot)