Yep. At work, for instance, I have 2 21" and 2 17". The 21" s are at considerably higher res than the 17s.
On most computers, you have the option of “mirroring” (the second monitor shows exactly the same thing as the primary; the computer thinks of you as having only one monitor) or using the monitors as an extended desktop, which is usually what you want. With the extended desktop–on a Mac, at least, and I’m pretty sure this is true for Windows as well–you pick one monitor to be the one that the OS treats as the “home” screen. On a Mac, that means the drive icons, trash can, and the overhead menu bar go there. On a PC, I would imagine that the home screen would hold My Computer, Network Neighborhood, the Recycle Bin and, if you’ve set it up to keep your other desktop icons automaticall rather than manually arranged, all your other desktop icons as well. But you can drag open folder windows and/or open program or document windows to the other screen. (You can even position a window so that part of it is on one screen and part on another, or stretch it to take up all of both monitors).
You can set the bit depth (# of colors) and resolution (pixels) for each monitor independently on-the-fly. (This can cause things that were on one monitor to end up halfway on the other, or cause the OS to reposition them when the space they used to occupy (say, at the bottom of your screen) ceases to exist.
You can position the monitors relative to each other, so that the primary is to the left of the auxiliary, or to the right, or above, or below, or connected diagonally at the corners, or even weirdly arrayed like partly overlapping. If you have three, you can set them up left-middle-right, or like an L, or top to bottom, or however you want them. (Mostly people set up the virtual arrangment to mimic the physical arrangement, of course).
You can plug a monitor into the VGA (or other type of monitor port) after the computer is booted, and it notices and adds the extra available space. You can switch one monitor out and put a differently sized one in without rebooting as well–the OS “loses” the first monitor when you take it off and makes an educated guess for the settings when the second one is hooked up. (occasionally it screws up and assigns an “illegal” refresh rate when you do that and you have to change the settings to make that monitor “legible”).
Programs that spew forth a zillion toolbars (can you say “Photoshop”?) are nice on a multi-monitor system since you can put all the toolbars on an auxiliary monitor and give your document a monitor unto itself. Photoshop and many other programs are clever about remembering where you put the toolbars and will put them there again the next time you open them.
One thing I find spectacularly impressive is that my PowerBook “remembers” which of several possible auxiliary monitors is which, and how I had them arranged. I have a Dell 19 inch Trinitron at work which I designate as the primary monitor, leaving the built-in TFT PowerBook screen as the auxiliary. (I put it on the right hand side of the desk and hook up a full-sized keyboard and mouse which sit in front of the big Dell monitor). At home, I have a Sony 15 inch Trinitron which I use as the secondary monitor–it sits on a little platform to the left of my desk, which is too small to hold a monitor and keyboard, so I use the laptop’s keyboard and the laptop’s screen as primary. So it’s the same operating system, the monitors hook up exactly the same, but if I’m at work the startup screen and drive icons and stuff come up on the external monitor, and when I’m at home all that stuff comes up on the built-in TFT monitor instead because somehow it KNOWS. (At the moment I have both the Dell and the Sony set to 1024 x 768 and 32-bit color, but I know from past experience that it also remembers different settings; if I change the Dell to 1280 x 1024, it will retain that setting but still run the Sony at home at the other setting).
The MacOS’s Control Strip comes with a button to change the bit depth and a button to change the resolution, and when you have multiple monitors, it recognizes them all and lets you change them independently without opening the Monitors Control Panel.
You can set different background pictures for each monitor using the Appearance Control Panel, and I’m pretty sure you can do so in Windows using the Display Control Panel item or by right-clicking the desktop and assigning different wallpaper to each monitor.
I’ve heard that DOS commands exist that can redirect the “attention” of the keyboard input from one monitor to another under DOS in old programs such as Autocad for DOS. (and you could make it render the pictures on one monitor while displaying the wire and controls and messages on the other).
In Windows, where the OS creates an opaque program window for every program, thus making it hard to have 4 or 5 different programs’ documents visible at the same time on a single monitor, multiple monitors makes it a lot easier to view an email message, a Word document, an Excel spreadsheet, a FileMaker database, and a DOS window at the same time without spending an hour de-maximizing and resizing and dragging. (Frankly, if I had Windows, I’d want at least three monitors for that reason alone).