Also
Old Friends - Kentucky based home of many old champions, and soon to be home for many more
http://www.oldfriendsequine.org/
The California Equine Retirement Foundation -- a good California one
http://www.cerfhorses.org/
Miracle Horse Rescue -- in Nevada, where they take mostly injured horses. They are currently in a fund raising campaign to outbid the slaughterhouse bidders at an upcoming auction
http://www.miraclehorse.com/
Glen Ellen Vocational Academy's Equine Retirement Foundation -- in Northern California, and endorsed by Harry Aleo and Lost in the Fog's connections
http://www.glenellenfarms.com/geva/index.htm
Most of the retirement farms will let you 'sponsor' a horse for $100 - $150 a month. With so many good organizations, and so many horses in need (and limited funds on my part), I could not decide which one to support. So I decided to give to a different one every month.