Achievements

Left 4 Dead will include Steam's achievement system, recording players' accomplishments and mishaps. Survivor players receive merits for accomplishments such as completing a scenario without using any health packs or helping a fellow injured Survivor. Demerits are given for mishaps such as shooting a teammate or walking into friendly fire.

The Infected receive merits for accomplishments such as killing or wounding a survivor, combined attacks on Survivors, or causing a swarm of AI-controlled Infected to kill a survivor. Demerits are recieved for mishaps such as harming a teammate, avoiding the survivors for a certain period of time, or purposely helping the survivors.

List of achievements
Here's a list of confirmed achievements. Valve is probably going to add more achievements later, when the actual game is released.

Zombie Genocidest Kill 53,595 Infected.

Pyrotechnician Blow up 20 Infected in a single explosion.

Akimbo Assassin Survive a campaign firing no guns other than pistols.

Dead Baron Survive the Dead Air campaign.

Towering Inferno Light a Tank with a Molotov.

Jump Shot Headshot a Hunter while he's leaping.

101 Cremations Set 101 Infected on fire.

Mercy Killer Survive the No Mercy campaign.

No-One Left Behind Beat a campaign with all 4 Survivors.

Blind Luck You or another Survivor take no damage after being vomited on by a Boomer.

My Bodyguard Protect any Survivor from an attacking Infected 50 times.

Tongue Twister Kill a Smoker who has grabbed you with his tongue.

Dead Stop Punch a Hunter as he is pouncing.

Drag and Drop Rescue a Survivor from a Smoker's tongue before he takes damage.

Cr0wnd Kill a Witch with a single headshot.

Untouchables No Survivors take damage after contacting the rescue vehicle.

Field Medic Heal 25 Survivors with a first aid kit.

Pharm-assist Give pain pills to 10 Survivors.

Helping Hand Revive 50 incapacitated Survivors.

Hero Closet Rescue a Survivor trapped in a closet.

Hunter Punter Shove a Hunter off of a pinned and helpless Survivor.

No Smoking Section Kill 10 Smokers as they are pulling helpless Survivors.

Do Not Disturb Sneak past all Witches in a campaign without disturbing one.

Man vs Tank Single-handedly kill a Tank.

Tankbusters Kill a Tank without it dealing any damage to a Survivor.

Safety First Play through a campaign with no Survivors taking friendly fire damage.

Unbreakable Beat a campaign without ever using a first aid kit on yourself.

Witch Hunter Kill a Witch without any Survivor taking damage from her.

Red Mist Kill 1000 Infected with a mounted machine gun.

Dead Giveaway Heal a fellow Survivor when your own health is below 10.

Ground Cover Save another Survivor from a Special Infected while on the ground.

Clean Kill Shove a Boomer and then kill him without him splashing on anyone.

Stand Tall Survive a campaign without being incapacitated.

Back 2 Help Leave a safe room to save an incapped teammate and bring them back safely.

Zombicidal Maniac Survive any campaign on Impossible.

What Are You Trying To Prove? Survive all campaigns on Impossible.

Lamb 2 Slaughter Incapacitate a Survivor after he has entered and exited a safe room.

Nothing Special Survive a campaign with no Survivors taking damage from Specials.

Burn the Witch Light a Witch with a Molotov.

Spinal Tap Kill an Infected with a single blow from behind.

Stomach Upset All Survivors complete a campaign without being vomited on.

Brain Salad Make 100 headshot kills.

Toll Collector Survive the Death Toll campaign.

Grim Reaper Survive the Blood Harvest campaign.

Barf Bagged Cover four Survivors with Boomer bile at once.

Big Drag Drag a Survivor x distance with your tongue.

Chain Smoker Constrict two Survivors on one life as a Smoker.

Double Jump Pounce two different Survivors on one life as a Hunter.

All 4 Dead Kill all four Survivors on one life as a Tank.

Dead Wreckening Dole out 5000 total Survivor damage as a Special Infected.