See Hidden Files in Snow Leopard's Open and Save Dialogs
Many of OS X's files and folders are hidden when working with the system--most of the Unix-only folders, such as /usr, /etc, and /bin, for instance. If you want to always see these hidden files and folders, you can do just that.
Most of the time, though, I don't want to see hidden files, as I don't do much with them. I really only want to see them when I want to open one of them--for editing in a text editor, for instance. For those situations, Snow Leopard offers a new solution to the hidden file dilemma: the ability to see hidden files quickly, but only in Open and Save dialogs.
In any Open or Save dialog in Snow Leopard, simply press Shift-Command-Period to display hidden files and folders. This command is a toggle; hidden files will be displayed as you navigate various directories in the Open or Save dialog.
To hide the hidden files again, just press Shift-Command-Period again. The setting is temporary, though--hidden files will always be hidden when the Open or Save dialog appears onscreen. (Somewhat obviously, this also only works if you've clicked the triangular button to expand the dialogs to show the folder browser.)
I covered this tip in my Six Snow Leopard tips video, but felt it was useful enough to share here, too--just in case not everyone watches the video. Thanks to Mac OS X Hints reader Magno Urbano for finding this hidden Snow Leopard gem.