You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad.Layout.Monitor

Define Monitor record. monitor can be used as a template. At least prop and rect should be set here. Also consider setting persistent to True.

Minimal example:

 myMonitor = monitor
     { prop = ClassName "SomeClass"
     , rect = Rectangle 0 0 40 20 -- rectangle 40x20 in upper left corner
     } 

More interesting example:

 clock = monitor {
      -- Cairo-clock creates 2 windows with the same classname, thus also using title
      prop = ClassName "Cairo-clock" `And` Title "MacSlow's Cairo-Clock"
      -- rectangle 150x150 in lower right corner, assuming 1280x800 resolution
    , rect = Rectangle (1280-150) (800-150) 150 150
      -- avoid flickering
    , persistent = True
      -- make the window transparent
    , opacity = 0xAAAAAAAA
      -- hide on start
    , visible = False
      -- assign it a name to be able to toggle it independently of others
    , name = "clock"
    }

Add ManageHook to de-manage monitor windows and apply opacity settings.

 manageHook = myManageHook <+> manageMonitor clock

Apply layout modifier.

 myLayouts = ModifiedLayout clock $ tall ||| Full ||| ...

After that, if there exists a window with specified properties, it will be displayed on top of all tiled (not floated) windows on specified position.

It's also useful to add some keybinding to toggle monitor visibility:

 , ((mod1Mask, xK_u     ), broadcastMessage ToggleMonitor >> refresh)

Screenshot: http://www.haskell.org/haskellwiki/Image:Xmonad-clock.png

• This module assumes that there is only one window satisfying property exists.
• If your monitor is available on all layouts, set persistent to True to avoid unnecessary flickering. You can still toggle monitor with a keybinding.
• You can use several monitors with nested modifiers. Give them names

• make Monitor remember the window it manages
• specify position relative to the screen