WAW, BO2, & MW3 Alternative to Steam. Awesome site to play online with Randoms or Friends.
Modern Warfare 2, Ghosts, and Advanced Warfare Alternative to Steam. You can play online with Steam version or download the Full Games and play online.
BO1 Alternative to Steam. Seems a little difficult to setup, but if you spend 30-60 minutes reading you should be able to get it up and running.
This video will give you a download link and instructions on how install Call of Duty 4 and play online.
A modded Call of Duty client based off the MW2 engine with all new features, weapons, perks, streaks, maps, QOL changes and more.