OS X can, to a limited extent, interact with background applications. For instance, you can move background windows around without activating them–just hold down the Command key and then click-and-drag on the background window’s title bar. You can then move the window around, and when you release the mouse button, the background application will not come to the foreground.
In some cases, though, you can do more than just move a background window around. You can, for instance, Command-click on links in Safari or Firefox to open those links without bringing the browser to the foreground. Even more interesting, though, is that you can activate contextual menus–in many applications–without bringing a background application to the foreground.
Why might you want to use this trick? If you’re copying a number of links from your Web browser into a TextEdit document, for instance, you can use this trick to save the time required to Command-Tab back and forth between the applications.
In order to do this, though, you need an actual two-button mouse, or a trackpad that’s set up to work as a two-button mouse. In other words, you can’t use a Control-click to activate a contextual menu in the background. Position your windows such that you can see the background window in which you’d like to activate the contextual menu, then just right-click and the contextual menu should appear.
This trick also might not work in all applications, though it worked in quite a few in my testing–Safari, Firefox, iChat, Terminal, and Mail all handled it fine. The Finder, however, was a bit strange. I wasn’t able to activate a contextual menu in the main portion of the window, but I could do so for items in the sidebar. The best way to know if a program supports this trick or not is simply to try it–if the program comes to the foreground before displaying the contextual menu, then you’re out of luck.