So this is based on my un-released "Project Cy" game.
It has built-in multiplayer!

Controls
WASD - Move
Mouse - Aim
LMB - Shoot
1-5 - Change Gun
Aim
Kill zombies!

Changelog
v 0.06
- Increased the player's speed!
- Added Ammo! (Used when firing all guns except for the tazer. Dropped by all zombies [20% Chance])
v 0.05
- Added new zombie (Robot)
- Added new gun! (Tazer)
- Full-proofed server
v 0.04
- Added crude main menu
- Added new zombie! (Physic)
- Bazookas can now kill players
- Optimizations
v 0.03
- Added Bazooka
- Added Explosions!
- Added new zombie! (Heavy)
v 0.02.5
- Made clients only run a server when needed.
- Switched to TCP, which allows for more zombies at the same time. (Increased zombie limit to 50)
- Respawn when dead.
v 0.02
- Added new zombie (Stricker)
- Made player move slower when shooting
v 0.01
- Initial Release