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Lightroom allows for two applications to be integrated as external editors. The first external editor slot will always default to Photoshop or Photoshop Elements (if installed), so you really can only control the addition of a second editor.
Faced with that limitation, Lightroom user Dirk Essl came up with a novel solution: Create an application that could be configured as a second editor but whose actual purpose is to provide access to as many editors as you wish. It is a simple solution and quite clever.
I've only experimented with the Mac version, but there is also a version for Windows XP and another for Vista. Each version is free (and use at your own risk). One aspect of Lightroom that I have been enamored with from the start is the community of users that have evolved along with it. I never cease to be amazed at how much time and resources people are willing to share with complete strangers. Thanks, Dirk!