I just started to use multiple monitors on Mac OS X (Leopard) and immediately encountered issues with Outlook for Mac OS X. When I open a new window, to write a new e-mail, it positions the window on the main monitor and not on the second monitor where I have Outlook open.
That becomes very annoying when you use Screen Sharing and look only at one monitor at a time.
A small tool, SizeUp, comes to the rescue. SizeUp lets me most of all move the current active window to the other monitor, using a keystroke. So when I open a new window and it appears on the wrong monitor I can easily put it into its place.
SizeUp also allows to size the currently active window to the right or left half so I can have two windows side by side on a monitor, which is great for copying comparing documents.