Here is how to play Left 4 Dead in a LAN with one or more Steam Accounts:
1) Download and install Left 4 Dead, either from Steam or a DVD, on all of the PCs you are going to use.
2) Download the Left 4 Dead Left 4 Dead Dedicated Server on one of the PCs.
3) Run the Left 4 Dead Dedicated Server and enter your settings (I would run it in LAN mode but it works either way).
4) Switch every Steam account to its "offline" status, providing you are going to re-use it/them.
5) Run Left 4 Dead on all of the PCs.
6) Go into the game menu and then the keyboard configuration - enable the console.
7) Go back to the main menu and press the Tilde ("~") key to open the console.
Type "openserverbrowser" to open the "classic" server browser.
9) Select the "LAN" tab.
10) Click "Connect."
11) Enjoy the game!