Killing zombies never gets old. Never. Film studios know it. Game developers know it. George A. Romero most definitely knows it. And judging by the plethora of Left 4 Dead 2 mods available out there for download, the modding community knows it too.
But how do L4D2 players know which custom maps are worthy of being coated in zombie blood, and which maps are better left dead and buried? Well, we've done the hard work for you. What follows are the top five custom campaigns we've found for L4D2. All free to download. And remember to *cough* gore hack *cough* before you play!

5. Left 4 Mario

So what could make the cutesy, colourful world of Super Mario even better? Zombies, of course! This fairly short custom campaign sees our L4D2 survivors tackle maps loosely based on the first three levels of the original Super Mario Bros game on the NES. And while the idea is undoubtedly ridiculous, it actually works. Players will climb through green pipes, jump over red shells, and shoot golden question-mark boxes in their quest to reach the flag poll at the end of each level.

There are a couple of small issues, including the fact that survivor bots just can't seem to handle some of the platforming sections. But so long as you are playing with human-controlled survivors, you should have a lot of fun transforming Miyamoto's NES masterpiece into an R-rated, undead bloodbath. Also keep an eye out for the dead bodies of Mario and Luigi (it's a me, dead Mario!). Download it from here.

4. Indiana Jones and the Temple of Zombies

This campaign is inspired by the Indiana Jones ride at Disneyland, California, and supposedly it's a 100% authentic recreation (except for the fact that there are no annoying tourists, just annoying zombies). The campaign features three maps: the first sees survivors "queue up" for the ride, the second is the ride itself, while the third is an escape scenario.

The whole experience only lasts about 20 minutes, but it's a fun trip. The ride itself is like an Indy-themed ghost train, complete with a rolling boulder and some cool Indy theme music. Of course, this is L4D2, not Disneyland, and the ride soon breaks down, the temple begins to collapse, and the survivors are attacked by hundreds of undead spawns of hell. But for a while there, it's almost like the survivors are on a nice, relaxing theme park holiday. And these guys deserve a holiday. They really do. Download it from here.

3. Dead Before Dawn - Director's Cut

This huge campaign is a L4D2 recreation of the film Dawn of the Dead (the 2004 remake, not the awesomely kitsch 1978 original). Just like in the film, the survivors must fight their way through the streets until they stumble upon a giant shopping mall which, unfortunately for them, is also infested with zombie scum... but at least now they can window-shop in between bouts of slaughter.

The campaign is actually a "Director's Cut" of an old Left 4 Dead campaign, but it stands the test of time. It's also one hefty beast - weighing in at almost 1GB (uncompressed). It features custom voice-acting, sound effects, textures, cut-scenes, and some maps even have multiple paths to completion. There are some very cool ideas here, including a section where you must stop zombies from getting inside the shopping mall, as well as a fully working kiddie amusement ride that looks like a miniature spaceship.

On the downside, the voice-acting can be hit-and-miss (these are modders, not actors, after all) and some maps are too repetitive. Still, the main attraction of this campaign are the shopping mall levels, and these do not disappoint, with lots of colourful and comical stores to maim zombies in, and even a "Real Zoey" strip-club (but no nude Zoey, unfortunately). Download it from here.

2. I Hate Mountains

This is another L4D2 port of an immensely popular Left 4 Dead campaign. The polish of this mod is simply astounding. All the maps ooze professionalism, and it's probably one of the few custom campaigns out there worthy of receiving Valve's official seal of approval.

The action takes place across a variety of settings, including a creepy forest, a creepy mansion, and, you guessed it, a creepy church. Granted, most L4D2 players are probably sick and tired of killing zombies in such clichéd settings, but the maps are innovative and original enough to keep you entertained. In fact, some sections are so beautifully designed (i.e. the waterfalls) that one could almost liken the setting to a work of art... that is, if hundreds of super ugly zombies didn't keep spoiling the view.
It all adds up to what is probably the best traditional L4D2 campaign out there. But if you like your campaigns a little stranger, then you'll love number one in our list. Download I Hate Mountains from here.

1. Questionable Ethics

This is one crazy campaign. On first appearances, Questionable Ethics looks deceptively simple: the action takes place in a series of basic square rooms, which are all painted a boring shade of dull grey. But it's what players discover inside each of these rooms that makes this campaign interesting - you will laugh, scream, and will probably go cross-eyed with sheer bewilderment.

The beauty of this mod is the surprise factor, there is just so much weird and wonderful stuff to discover, and I don't want to give too much away. But to give you an idea of the type of ridiculousness you will come across, this is what players face at the end of the second map: the survivors are told to descend down a huge vertical tunnel by precariously jumping onto thin drainage pipes, while at the same time avoiding cars as they fall from the ceiling. Yes. Cars falling from the ceiling. I told you it was weird.

The campaign is also quite difficult, with an epic number of hordes, tanks and witches to contend with, making it the perfect challenge for L4D2 veterans. It's also the only mod on this list that actually made me jump off my seat in fright (yes, embarrassing I know). Questionable Ethics may not be as beautiful or technically brilliant as other mods on this list, but in terms of originality and pure fun, this campaign takes the cake. Plus, it's got lasers. Yep, lasers. So play it now - download it from here.