I am big fan of multiple monitor system. Just curious to know what other devs are using.
Here is my desk.

Ubuntu laptop has nothing to do with monitors here ;)
I've been using 2 24" plus my 13" laptop screen, but am seriously considering just going to one larger widescreen monitor.
You can never have too many monitors. Time is money. If your going rate is $50/hr then that means the company you work for pays $100/hr for your time and clients pay $200/hr for your time. At that rate, all a monitor has to do is save 1-2 hours of your time in order to be worth the price (depending on the monitor and keeping in mind electrical costs as well as the computing power necessary to drive that many displays).
I used to have 4 monitors for my desktop plus 7 mobile devices nearby. These days I have a giant 4K TV as my main display (which replaces 4 monitors) plus 4 mobile devices. The 4K TV was purchased to improve my photo editing capabilities.
Usually I have a photoshop document on one display, a coding window on another, the resulting page on another monitor, and a style guide or code reference in another window.
On my second virtual desktop I have a console for ssh, one for logs, one for compiling / build errors, and one for general use.
On my third virutal desktop I have work IM, work email, spreadsheets, and OneNote.
On my fourth virtual desktop I have a music player, a browser with forums & news, personal email, and personal IM.
My fifth virtual desktop is reserved for overflow from the other ones.
The mobile devices are for mobile app development and/or for taking with me to meetings. In the case of the Surface Tablet it's for specialized tasks like drawing, sketching ideas, or note taking.
OLD:
2 desktop monitors, 2 USB monitors, 3 phones, 3 tablets, 1 notebook.
NEW:
1 50" desktop display, 2 notebooks, 1 hybrid, 1 tablet
Not even an option in the poll for 1?
I've worked on a number of different setups, including a 5-screen battlestation at one time, but I've never been more productive than using my macbook air's screen alone :O
What I do instead is set custom triggers for every app I need to use, so from inside any other application I can press 3 keys and immediately see whatever app I want to use :D Being able to launch programs that fast makes the single screen more usable!
I use a Macbook Pro with the screen extended to an iPad Mini via Duet and usually need nothing more, but when I do need more management I use the workspaces feature built into OSX. It's a perfect balance of portability and productivity (most of my work has been remote, lately - so I don't sit at an office). If I need to get a broader look at a lot of data (say, on a dashboard), I airplay directly to my smart TV.
I can make due with 1 monitor if I have to, but 2 is the sweet spot for me. The stuff I'm actually working on on one screen, any documentation/Google/examples on the other.

2 for systems administration / dev with 1 on the wall for Google Analytics
That is indeed a standing desk from IKEA. It is the manual crank one rather than the automatic lift. I love the desk.
Joe Clark
Full-stack developer specializing in healthcare IT
I have one large 27" monitor connected to my laptop. I find having more than one monitor is more distracting than I prefer. If I were to spring for a decent monitor stand, I might like to try two 27" monitors vertically. But for now, just one.