Mac: By itself, the Command-Tab Application Switcher can move back and forth between open programs. With LiteSwitch X installed, you can hide, close or force quit applications, drag files onto them for opening, and even include background applications in your switching.
If nothing else, give LiteSwitch a try for the ability to quit and hide applications from the Command-Tab menu—that alone saves you the trouble of mousing around, and you'd have to switch over anyways to close something down with a keyboard shortcut. The other functions are pretty neat in their own way, too, and you can see file opening and contextual menus working their way into a more advanced workflow with LiteSwitch at the core.
LiteSwitch is a free application while its 2.7 version is in its beta phase—there's not much more information available about what happens after this, though the full version will cost $US15 at some point. In the meantime, enjoy your better Application Switcher, and tell us how you use it in the comments.