Dual monitors, I have one mounted above my main monitor. I work in media and often with videos or pictures so it's useful to pull windows into another screen to browse through or view them. For other work it's just nice to have my Spotify or other programs up and away so they're not quite so distracting! Multiple monitors kind of give me a physical representation of the word "digital workspace" :)
@eddieaich I'm definitely much faster with a superwide monitor! I'm also using this tool called, Spectacle for Mac, with which I can divide the superwide monitor into two plus the monitor of the MacBook, ta-da, I have 3 solid workspaces that I can control simultaneously. Reading a related previous email, while typing a new one is much faster with multiple monitors instead of switching tabs back and forth.
But I strongly believe if I have any more monitors, I'd definitely get lost in them :D