in Windows 8 you can stick a Metro app to a side of the monitor so it runs on the side.
The problem is, that you can do that apparently only on the primary monitor. Is there a way to display it on the secondary monitor?
You will now have the target application snapped into small mode on the desired monitor, with the rest of the monitor used as desktop space. I like to do this with skype.
Note, you can still only have one "metro" monitor. Once you move a metro app to the second monitor, the start screen will appear on that monitor as well, and any other metro apps you open will appear on that monitor. If you try to move a metro app back to the main monitor, the sidebar application will be moved from the second monitor to which ever monitor you just moved the other metro application to. This is a big pain that I hope they fix in 8.1.