require 'tk'

f = TkFrame.new.pack
scr = TkScrollbar.new(f).pack('side'=>'right', 'fill'=>'y')
list = TkListbox.new(f).pack('side'=>'right')

list.yscrollbar( scr )
for i in 0..20
  list.insert(i, 'item ' + i.to_s )
end

label = TkLabel.new(nil, 'text'=>'no item is selected').pack
list.bind('ButtonRelease-1', proc{ label.text 'index ' + list.curselection.to_s + ' is selected' })

Tk.mainloop