The other day I got a new monitor in the mail from NCIX (who I highly recommend if you haven't heard of them). I realized that having two monitors at my last job gave me much more productivity, but since I work at home now I should have two monitors here. That's my excuse and I'm sticking to it.
I wrote a post less than a year ago on how to get dual head going with Compiz Fusion and back then it was really really annoying. Since then we have the nvidia-glx-new package right in Ubuntu, so there's no more having to shut down X in order to install the driver. Awesome.
So yeah, all you have to do is plug in your monitor, have nvidia-glx-new installed (assuming you're running an Nvidia card, no clue how to do it with ATI), open System->Administration->Nvidia X Server Settings, click X Server Display Configuration and enable things in there. For me, I had to click Detect Displays and click the monitor that is disabled. I clicked Configure, selected TwinView and off it went, working perfectly.
There was one last small issue. My desktop background was centred :(. So half of it was on one screen and half was on the other. There are two real fixes for this. If you just want the same image on both sides, you right-click the desktop (in Ubuntu), go to "Change Desktop Background", and set Style to Tiled. Done!
If you want to have two different backgrounds, the easiest option is probably just to take your current image, add some space to the right of it and paste a new image there. Then save your huge image as a file and select that as your desktop background.
On another note, having two screens like this for web development is awesome. I can fit a browser window, two IM windows, GVim and two terminals all without overlapping. Maybe it's time to try a tiling window manager!