If you can pick up an old copy of half life for 2-3 dollars US you can download counter strike, day of defeat, team fortress for free from steam. These are the old version of the games though not the newer ones. If you’re looking for the newer versions you will have to pay separately for each game.
first of all let me clear that You are telling to download the game in Offline ??? i think you need internet ( its call online mode ) to download anything .!
second thing if you are looking for game for free not to buy the game then you can google it there are lots of links of direct as well as torrents downloads. but personally i prefer to buy the game from the steam coz you can play it on Steam network ..!