counter strike source that you can just download and play?

there is no free CSS
but you can try True Combat:Elite, its almost like CSS
www.truecombat.com
its a MOD for Wolfenstein:Enemy Territory so you'll have to dl that first


Download and install this first.
http://www.truecombatelite.net/component/option,com_remository/Itemid,40/func,select/id,41/
Now download this update, and install it to your Wolfenstein:Enemy Territory folder (Default should be:C:\Program Files\Wolfenstein - Enemy Territory)
http://www.truecombatelite.net/component/option,com_remository/Itemid,40/func,select/id,45/
Now download and install True Combat:Elite
http://www.truecombatelite.net/component/option,com_remository/Itemid,40/func,select/id,37/