Good news: now, even those who haven't pre-ordered Left 4 Dead can get their mass zombie killing on ahead of the full game's launch. As Shacknews notes, Valve has released the Left 4 Dead demo to the public via both Steam and Microsoft's Xbox Live Marketplace (for Xbox 360 owners).
Steam users can play by clicking the "Install Demo" link on this page. The installation weighs in at a hefty 3.2GB, though, so folks with 128Kbps DSL connections should probably abstain. The demo includes two levels from the "No Mercy" campaign, allowing up to four players to shoot their way from the roof of an apartment building through to the bowels of a derelict subway network.
The full version of Left 4 Dead is still due out on November 18 for the PC and Xbox 360.