This is a useful configuration for a distraction-free note-taking system. Often you want to quickly note something but don't want to break your flow too much.
The result will be a single key-combination that will pop a frame, allow you to write in a task, and have it be written to your default org-mode file.

Nearly all of the meat of this configuration comes from Phil Windley's blog post. I assume you already have experience with org-mode.

;; org-capture code for popping a frame, from Phil Windley

;; We configure a very simple default template
(setq org-capture-templates
      '(("t" "Todo" entry (file "")  ; use the default notes files
         "* TODO %? %i %t")))

(defadvice org-capture-finalize 
    (after delete-capture-frame activate)  
  "Advise capture-finalize to close the frame"  
  (if (equal "capture" (frame-parameter nil 'name))  

(defadvice org-capture-destroy 
    (after delete-capture-frame activate)  
  "Advise capture-destroy to close the frame"  
  (if (equal "capture" (frame-parameter nil 'name))  

;; make the frame contain a single window. by default org-capture  
;; splits the window.  
(add-hook 'org-capture-mode-hook  

(defun make-capture-frame ()  
  "Create a new frame and run org-capture."  
  (make-frame '((name . "capture") 
                (width . 120) 
                (height . 15)))  
  (select-frame-by-name "capture") 
  (setq word-wrap 1)
  (setq truncate-lines nil)
  ;; Using the second argument to org-capture, we bypass interactive selection
  ;; and use the existing template defined above.
  (org-capture nil "t"))

Now we add the binding to your (hopefully existing) call to the xmonad additionalKeysP function, which uses emacs-style notation for the keybindings.

popCaptureFrameCommand = "emacsclient -n -e '(make-capture-frame)'"

myConfig = desktopConfig
       -- your existing config here
    `additionalKeysP` [("M-o c", spawn popCaptureFrameCommand),
                       -- probably more existing bindings...

Now you can type M-o c, you'll be popped into a capture buffer, then C-c C-c will save and file it and close the window. It'll appear as a top-level heading in the file. You can change the template definition if you are more OCD-minded but I find that this simplistic configuration works and stays out of my way.