Alien Swarm

It's a Valve game. Except, it's free and not first person! I know, I had to sit down for a few minutes myself! It's actually a update of an old Unreal Tournament 2004 mod, done by the original developers in their spare time from working on Left 4 Dead and Portal 2.

Classic Aliens
For some reason, rougher versions of the Aliens from before Valve hired the dev team are present and working in the game, including an Alien Queen boss.

Zombie
There is full sourcecode and sound definitions for a zombie enemy (described in the source code as a "Swarm Civilian corrupted by SynTek chemicals into being a crazed zombie like thing", but curiously enough no model or sounds and he can't be spawned in game.

Code can be found at steamapps\common\alien swarm\sdk_src\game\client\swarm\asw_zombie.cpp and  once you've installed the mod SDK.

Heavens only knows how this didn't get in the game. Everybody knows that Valve luuuurve their zombies.

Leftover Text
Pretty much all the UI strings from Left 4 Dead 2 are present, with a find/replace to replace all instances of "Left 4 Dead" with "Alien Swarm". This leads to some amusing unused strings... "L4D2_Game_left4dead1"						"Alien Swarm 1" "L4D2_GamePlaying_left4dead1"				"Playing Alien Swarm 1"

"L4D2360_NotJoinable_OtherTitle_left4dead1"	"Playing Alien Swarm 1" "L4D2360_JoinError_OtherTitle_left4dead1"	"Your friend needs Alien Swarm\nso that you could play together\n"