There is a beta version of Counter-Strike 1.6 available for testing. This beta introduces in game advertising. For the duration of the beta default Valve ads will be used, once the feature is released a wider range of advertisements will be present.

Install instructions.

To run this beta version you will need to add -beta cs to your command line when you start Steam. This can be done by opening up the Windows shortcut you use to launch Steam and adding -beta cs right after Steam.exe. Please report any bugs or issue you find in this article.

You should also disable Steam from running automatically when Windows starts, to prevent it from running the non-beta Source Engine. To disable Steam from running automatically

Valve wiki