Children and Nature Network  (This website, affiliated with
     author Richard Louv, has the best and broadest information
     about the "Leave No Child Inside" movement and the

     research behind it.  Highly recommended.)

National Wildlife Federation's Green Hour  (Good information,
     tips, and blogs on this site)

Natural Learning Initiative  (Based at North Carolina State
     University, this initiative features exceptional expertise in the
     design of naturalistic play areas.  It is led by noted landscape
     architect Robin Moore.)

Children, Youth, and Environments  (Based at the University 
     of Colorado - Denver, this initiative publishes a fine journal

     and sponsors an interesting list serve.)

Landscape and Human Health Laboratory at the University
of Illinois at Urbana-Champaign  (great research

     information on the values of green space)