Shoutbox

As the instructions explain on its download page:

1. Click on the script icon in the contact list of Windows Live Messenger
   
2. Choose 'Random phrase' from the script menu
   
3. Choose 'Open' to open the configuration window (which you need to do after installing because you first need to add phrases to choose from)
   or
   Choose 'Reload' to change your personal message to a random phrase from the list

But note that in Windows Live Messenger 2011, the script system to set a personal message is kind of buggy and might not work. It does work properly in Windows Live Messenger 2009 though.

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If you don't see the script icon in Messenger, you first need to enable it:

1. Press <F10> to enable the menu bar in the contact list
   
2. Click on the 'Plus!' menu
   
3. Open the Messenger Plus! preferences by clicking on 'Preferences & Options'
   
4. Navigate to the panel 'Appearance' > 'Contact List'
   
5. In there, enable the option 'Add Messenger Plus! buttons in the contact list and the contact cards'
   
6. Click 'OK' and restart Windows Live Messenger