SAS Zombie Assault 2 - Insane Asylum, is an awesome flash game in the same vein as Call of Duty: Word At War's Zombie mode.
You begin in a small space, earning cash by killing zombies and unlocking better weapons, more areas of the map, and various power ups.
Zombie Assault 2 ups the usual ante by featuring persistent upgrades provided you register, as well as having a number of different enemy types, nearly 20+ weapons, and just all around fun game play.
The weapons sound and feel good, and the overall execution of the game is just really solid- it's not revolutionary, but it's the best of it's type that I've played.
You can play it here.
Want to find other awesome online zombie flash games? Check out my list.