Plutonium is a custom client and loader for Black Ops 1 and Black Ops 2 . It is not strictly a "DLC unlocker" because it operates outside the official infrastructure.
These are small .exe files (like "Black Ops 1 Unlock All Trainer") that you run after launching the game. They scan the process memory and flip the DLC flags.
There are two primary ways players currently access DLC without official purchase: