With synergy, all the computers on your desktop form a single virtual screen. You use the mouse and keyboard of only one of the computers while you use all of the monitors on all of the computers. You tell synergy how many screens you have and their positions relative to one another. Synergy then detects when the mouse moves off the edge of a screen and jumps it instantly to the neighboring screen. The keyboard works normally on each screen; input goes to whichever screen has the cursor.
I highly recommend setting up dual monitors if you are a developer. I did it this summer after purchasing a 19" Dell Trinitron for $150 from eBay. If you're buying off eBay, make sure you know exactly what monitor you're getting. The first one I bought was a 21" Sony that sucked. Luckily, I was able to re-sell it fairly easily. Now, I actually believe that my two 19" Trinitrons might be better than the 23" Cinema Display I long for.