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Downloading and installing Wolfenstein - Enemy Territory is very easy. There are many websites through which you can download it but I suggest getting it from enemy-territory.4players because they provide the game and all the patches in only one install file. You'll avoid the hassle of downloading the patches. Just go to enemy-territory.4players and and register. Once registration is complete, go to Downloads and click on Main (ET). To download the Windows version click on the first line. That will download the full package with patches.

You'll also find Linux and Mac version somewhere further down as well as the source code