Curse Village

Curse Village is a very professionally done zombie survival game, with far more options and re-playability then many of its peers.
A comic introduces the story- you're some sort of semi-physic government agent sent in to defend a village from a zombie attack.
From there, you're introduced to the main mechanic- zombies run at you from the left, your shoot/stab/burn/destroy them before they reach you.
A whole host of weapons, upgrades, traps, and other survivors elevate this experience for it's 30 levels, and there are enough variety in the enemy zombie types to keep thing interesting.
Curse Village shares a lot of similarities with the also excellent Last Stand series, so that would be worth checking out if you enjoy this game.
You can play Curse Village here.

Want to know about more sweet ass online zombie games? See my list of the very best.