Cyberduck 3 will feature Quick Look, as known in the Finder.app since Mac OS X 10.5. You can toggle Quick Look in a Cyberduck browser for any file using
Command-Space Bar. A preview is rendered depending on a Quick Look Plugin available on your system for the given file type. Many file types like different image formats can be previewed with the bundled plugins in OS X and HTML is even rendered in the Quick Look preview panel.
Check this out in the the latest nightly build. Mac OS X 10.4 or later required.