• Plutonium (World at War, Black Ops, Modern Warfare 3, Black Ops 2)
• Alterware* (Modern Warfare 2, Ghosts, Advanced Warfare)
• XLabs** (Modern Warfare 2, Ghosts, Advanced Warfare)
• Boii** (Black Ops 3)

* Picked up where XLabs had to stop. ** Shut down via C&D letters.

Since Activison doesn't want to patch their older CoD games on PC community made clients are basically a must for those games. Not only are those adding patches against RCEs but also add a lot of customization, modability and dedicated servers. Some are even adding custom maps to it.

I also used to play Venice Unleased and Project Rome for BF3 and BC2.