HDR Lighting in Maya using HDR Shop, and the LightGen Plugin: The barebones approach to quickly get started with HDR lighting.
First thing's first, you'll need to download a free copy of HDR Shop from Paul Debevec's HDR Shop page. (Click on Download v1) You'll also need the "LightGen" Plugin found under the Plugins section of his website.
Create a directory on your computer called "plugins" in the same directory as HDRShop.exe. Copy the file "lightgen_plugin.exe" that you just downloaded, into this directory. When you start HDR Shop, the option "lightgen_plugin" should appear in the "Plugins" menu.
You can create your own image to use as an HDR light probe or use one of the free maps on Paul Debevec's site. If you download one of his, make sure to download a .hdr file.
Now load the lightprobe into HDR Shop.
Then select Image>Panorama>Panoramic Transformations from the main menu.
Select the lightprobe image as the Source Image and select which format it is in (if you are using an image of a mirror ball then select "Mirror Ball"). In the destination section on the right, select "Latitude/Longitude" and select a resolution of 128 x 64. Higher resolution isn't neccesary and it only slows down the light generation process. Hit ok to perform the conversion.
A new image in latitude/longitude format will now be displayed.