DBG = Taking advantage of a L4D glitch that allows everyone free access to campaign achievements by starting and finishing the finale only.
It's done by having one person create a lobby, start the game, invite the rest of the party in then back out onto the dash for a reinvitation. The only catch is that everyone has to start at the dash and if your character ever goes idle in the game you won't get the achievement even if you survive.
JG = An exploit on the map that allows players to evade hordes, special infected and glitch kill Tanks hence the name "Jesus Glitch".
No Mercy has the Jesus Room which you can attract and trick a horde into breaking down a door which will allow you access into a room. It's much easier to watch done than explain and YouTube has tons of videos on it. The programming is flawed so all zombies, special infected, Tanks and such can't run up the steps to gain access into the room so long as you hug the wall and remain there. Every so often two or three might drop down but they rarely run in. They just stand there staring at you in awe giving you enough time to kill them.
It isn't perfect though because the glitch will sometimes kill the second Tank, but freeze the game. The music will continue playing and you'll have to restart.
Dead Air has the Jesus Plane in which you can use a physics glitch on the gatling gun to slingshot your team on top of the plane where only Smokers can get to you. You basically crouch melee the gun until the bounce is adequate and jump on top of it. Once on top you simply crouch melee together in the middle of the plane to render Smokers useless then run to tail of plane. You'll phase right through it without taking damage right in front of the opening.
Death Toll has the Jesus Bolder/Rock/Whatever the hell you want to call it in which your team leaps on top of these rocks right next to the pier. It's so high off the ground and water that again only Smokers can get to you. Crouch melee to deal with them and you can kill Tanks yourself at your own discretion. Add the ridiculous amount of Pipe Bombs you usually find in the house and it's a guaranteed achievement.
Blood Harvest has the Jesus Tractor... but I'm not going to bother explaining that retarded glitch and not because someone already described it a few pages ago. The reason I'm against it besides the high level of difficulty required for pulling the jump off are the limitations of the glitch itself. Also the abundance of special infected and hordes auto-spawning whenever someone enters the cornfield. It's such a small roof you can only fit two people on top of it anyway. Unless you can pull it off consistently or are particularly in a masochistic mood I wouldn't try it.