In this tutorial i will show you how to light a scene with an HDRI image and then composite the shadow onto a back plate in GIMP. This is a great method for those sorely missing the "shadows only" option in Blender Internal.
Check out ben-blease.tumblr.com for WIPs, finished projects, wireframes, and tutorials.
Check out smcars.net's repository of HDRIs at http://www.smcars.net/forums/smcars-hdri/
And you can download the backplate and HDR images used in this tutorial at http://www.smcars.net/forums/smcars-hdri/38200-hdri-package-002-a.html
background image: http://smcars.net/HDRI/002/big/IMG_4825.jpg
Thanks for watching!