Read all about it here: Finer Things in Mac

In Snow Leopard (OS X 10.6+), if you need to open a hidden file, all you need to do is type Cmd-Shift-. (that’s a period) in the standard Open File dialog. All hidden files will be displayed.

This is a toggle, so typing the shortcut a second time will hide the hidden files again.

This site is all about finding the somewhat subtle things about OS X that make it great. If you haven’t gotten snow leopard yet, what are you waiting for?