* 15" Retina MPB at 1920x1200 (iTerm 2 with either 3 or 4 panes)
* 27" Thunderbolt (two documents side-by-side, currently tolerating Atom)
* 27" iMac in target display mode (browser and dev tools).
And that's just the three spaces that I do most of my work in. I've got other spaces set up for general browsing, email, image editing, chat, etc.
It's probably excessive for most people. But I need to see all of these things. If I can't have them all up at once, I have to do a lot of switching between windows and spaces, and that takes a time. It's small amounts, but it adds up. I rely on tools like Gulp and BrowserSync to help me save small amounts of time. Staying focused by keeping my work in front of me really does help to keep me focused.