Where can I find the latest release of Darwin?
Unfortunately, ISO files are no longer made available. From http://en.wikipedia.org/wiki/Darwin_%28operating_system%29#History:


  Up to Darwin 8.0.1, Apple released a binary installer (as an ISO
  image) after each major Mac OS X release that allowed one to install
  Darwin on PowerPC and Intel x86 computers as a standalone operating
  system. Minor updates were released as packages that were installed
  separately. Darwin is now only available as source code,[4] except for
  the ARM variant, which has not been released in any form separately
  from iOS. However, the older versions of Darwin are still available in
  binary form.
  
  [4] http://lists.apple.com/archives/Darwinos-users/2007/Oct/msg00006.html