iTunes has a neat feature that lets you float the miniaturized (MiniPlayer) version of iTunes on top of other applications so that you can use other apps and still see/access iTunes without having to bring it to the foreground. To enable this feature go to the Preferences in iTunes and select the Advanced tab. Near the bottom of the tab, check the box next to Keep MiniPlayer on top of all other windows. Once this box is checked, when you are looking at the MiniPlayer (hit the Maximize button in the iTunes window) it'll float on top of all other windows even when it is not the active application.