I'm probably the last person to find out about this, but Picasa (free) now shows thumbnails for PSD and raw file formats.

I don't use it to edit photos, but it's astonishingly fast at rendering thumbnails and letting you browse through many folders of photos at the same time: there's no need to traverse a tree view and open folders one at a time.

In that regard it makes Adobe Bridge and the ordinary OS X file browsers look pretty lame, unless I'm missing something.