Valve's released a Left 4 Dead 2 toolset that allows the PC community to build maps for each other.
But if you can't be bothered, simply install the add-on support package and reap what they sow. This is a meaty download, but once up and running even Left 4 Dead 1 custom maps will be playable.
Designers, on the other hand, need the Authoring Tools. These behave like the L4D1 toolset but with added features. One of those is Gamemode Logic, which cleverly adapts maps to modes being played; another, Director Logic, knows a good team of Survivors when it sees one and so ups the challenge for them.
Rounding the package off are Scavenge mode support and a full, maybe lovingly illustrated manual to help you get going.
Head over to the Left 4 Dead blog for download locations and other helpful links.