I have a computer with two DVI outputs in which i have plugged one 24" and one 20" monitors. I'd like to plug one more monitor to my computer. What do I have to do?
Easiest solution - If you have any other output such as VGA, get an adapter and use it.
If not, get a second graphics card and use that - you can just get the cheapest one that will fit - it should do the job.
Edit - As Shoeless said, you must use same manufacturer for Windows Vista, but I do not think there is such limitation in either Windows XP or Windows 7. If you are using *nix or anything else, I do not know in advance if it will work.
NOTE: Depending on your OS, when using a second display card, it might be necessary to use an identical chipset- so if your curent card is an nVidia, make sure your second card is an nVidia... or ATI, etc.
I learned this the hard way. Theoretically you could get them to cooperate using XP drivers, but really, not a good idea.
The newer ATI HD5xxx series supports a larger number of displays. Normally three on the usual cards (check the connectors, some single-slot cards won't have enough digital connectors).
Depending on what you want to use it for, I haven't tried any USB adapters, but I imagine they would not be amazing quality compared to another graphics card, so if you are only going to be using it for something like web browsing or emails, then a USB adapter may be perfect for you, however if you plan to do anything which requires a lot of data to be processed, for example if you were playing a game or watching a video then a 2nd graphics card is the way to go. However your motherboard will need to support dual graphics card, and support SLI (NVidia) or CrossFire (AMD) to allow you to use 2 graphics cards simulataneously, and obviously the cards would have to support this aswell. Which is why you can see, a USB adapter would be easiest for something small.