#9
Tried this out very briefly the other day and shows some promise. It uses the old Unreal 2004 engine (the menus look identical), and plays quite well.
You get waves of zombies hitting you, with short breaks to level up and purchase weapons and ammo in between before the final boss zombie on the last wave.
It also has a very basic objectives mode, similar to L4D, although not many maps for that one. It does, however look like mods for it are plentiful, which is promising news. Further investigation required, but worth a look.