But not the official kind (you'll have to wait for Left 4 Dead 2). The beta version of custom content tools has been available for some time, but this week Valve will release the official version of the tools, along with ingame support for custom levels.

We've long believed that Left 4 Dead should fully support community content, and we've been hard at work enhancing the UI and matchmaking systems to provide full support for your creations. The latest update, which will be available via Steam [this] week, is focused on custom campaigns.

I can imagine how many mod-makers are champing at the bit to release new levels. I expect a flood of new maps as soon as the update is available. I'm not saying they'll be good, but at least they'll be new.