The invisible files hiding on your Mac are, for the most part, invisible for a reason. Since there are times when you might need to see those hidden files and folders, Mac OS X 10.6 makes it easy to do -- assuming you are viewing an Open or Save dialog.
To temporarily see the hidden files and folders on your Mac, Type Command-Shift-. (that's the period key) after invoking an Open or Save dialog. You'll be able to see all of the hidden files and folders on your Mac, but only in the dialog, and only as long as that specific dialog is open.
Snow Leopard lets you temporarily view hidden files.
This is a handy way to find and edit files that otherwise aren't visible, or if you need to get into documents that are stashed away in an invisible directory.
If you prefer to see all of your invisible files and folders all of the time, take a look at TinkerTool. That application lets you do far more than show hidden files, and it's free.