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.