While the campaigns that shipped with Left 4 Dead were all pretty good, you do get sick of them after a while. Which is where custom, user-generated campaigns come in.
Next week, Valve will be updating Left 4 Dead so that these custom missions - like the Resident Evil 3 mod we showed you earlier in the day - are both more prominent within the userbase and are easier to download and play.
Firstly, with this update map creators will be able to package their content into a single file (a .VPK). This will make displaying, downloading and installing the campaigns a cinch for other users. The game's matchmaking system will also be updated to reflect this, with "extensive support for custom add-on campaigns"..