To hide an application while command-tabbing between your open apps, simply hold down "H" while that application is highlighted in the dock. Similarly, hold down "Q" to quit the selected application. This is a great way to hide or quit apps without making them active first.
The only thing to watch is that there's no "undo" for this -- once the app is hidden, it's hidden until selected. And when you release the "Q", the selected application quits immediately.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011001232308329