So I'm working on one.
require 'wnck'
Gtk.init
w = Gtk::Window.new
b = Gtk::HBox.new
w.add b
Wnck::Screen.default.force_update
Wnck::Screen.default.windows.each do |win|
but = Gtk::Button.new
but.add Gtk::Image.new win.icon
b.add but
end
w.show_all
Gtk.main