I don't know of any third party control panels/extensions to do so in Finder, but ResEdit shows invisible files in its open dialog (and can also change the invisibility flag of files). There are also a few file browsers that can view invisible files. One is LetsSee, personally I don't like the app's interface and speed, but it has a visibility filter option that allows you to see invisible files (classic version at http://www.irony.biz/letssee/letssee.sit (1.3 MB)).