In addition to the big sale, there is a free multiplayer weekend going on right now for Call of Duty: Ghosts. I do believe it shows up in your Steam Games' list if you don't own it so that you can download it right there. It's like 40GB, so if you want to play this weekend, you probably shouldn't wait to get started.
If you want any of the present or past Call of Duty games, right now is probably your best bet. Head on over to Steam to start browsing through the games featured in the Call of Duty franchise sale.