Hammerspoon (http://hammerspoon.org) script to switch to next window of frontmost app in OS X

nextWindow.lua

-- Window Manipulation

-- Bind alt-Tab to show next window of current application

hs.hotkey.bind({"alt"}, "Tab", function()
	local app = hs.application.frontmostApplication()
	local windows = app:allWindows()
	
	local nextWin = nil
	
	-- Finder somehow has one more invisible window, so don't take it into account
	-- (only tested on Yosemite 10.10.1)
	if app:bundleID() == "com.apple.finder" then
		nextWin = windows[#windows-1]
	else
		nextWin = windows[#windows]
	end

	if nextWin:isMinimized() == true then
		nextWin:unminimize()
	else
		nextWin:focus()
	end
end)

I haven't checked to be sure, but I am tempted to guess that Finder's weird invisible window might well be the desktop

Thanks for the comment!

Even if that's the case unfortunately it doesn't respond to the focus() call. (Maybe that's how the desktop should work.)