Two in, Two out
Chimera is a starter-level dual extruder package. More compact than mounting two V6 hotends side-by-side, Chimera is the perfect way to open your printer to the possibilities afforded by multi-material printing. With two separate V6 Heater blocks, Chimera lets you print with two materials at different temperatures, allowing you to use dedicated support materials such as Scaffold.
Two in, One out
Cyclops is a more advanced multi-material system for higher quality printing. Because Cyclops feed both filaments through a single nozzle, it eliminates oozing problems with traditional dual-extruder setups. You'll still need to tweak your settings to get perfect prints, but Cyclops blows away the competition once you get it dialled in.
Four in, Four out
Not for the faint of heart, the Kraken lets you print all the impossible things. With four distinct hotends, Kraken gives you the absolute maximum amount of freedom to print anyway you want. Multi-color printing, multi-material printing, or both at once, the world is your oyster. Just note that getting Kraken working is a herculean task on its own. It works, it'll just take you working with it for a while to get good prints.