Achievements

Left 4 Dead includes achievements for Steamworks and Xbox LIVE. There are 51 achievements in all for the PC version (not including the 6 from Survival Mode), but only 50 for the Xbox 360 version (not including the 6 from Survival Mode)—the Outbreak achievement is not in the Xbox 360 version of the game.

Note that none of these achievements can be achieved with cheats turned on. For help on how to obtain some of these achievements, see Obtaining achievements.

Cheats can be used to unlock achievements, all you have to do is kick all the players through the console then turn on sv_cheats 1 and noclip 1, then fly to the end of the level then turn off noclip and sv_cheats. Repeat this through all the levels in a campaign and achievements can be easily unlocked through this. ie, if you want man vs tank kick all players then noclip in the air with sv_cheats 1 look directly at your feet and z_spawn tank then turn off sv_cheats 0. The tank will not be able to attack you even with the rock throw and he just runs back and forwards under your feet.

Note that achievements cannot be reset so if you decide to cheat then there is no way to get the achievement later without cheating.

Survival Mode
All of the Survival Achievements are not present on the Xbox 360 version of the game since it is free DLC.

Left 4 Dead 2
The Achievement Drag and Drop has been spotted from gameplay footage

The Clean Kill Achievement has also been found in the videos of Left 4 Dead 2 gameplay