gdnsd users:

• Wikimedia Foundation - gdnsd is the DNS server for Wikipedia!
  • Public Stats
  • Public Configuration Data / Templating
• Miraheze uses gdnsd, with the config on Github
• OpenStreetMap is using gdnsd, and their Chef cookbook for gdnsd is available!
• Scale Engine is using gdnsd for their Video CDN: Blog Post
• Apparently the default geodns routing for ftp.freebsd.org and download.freebsd.org via the subdomain "geo.freebsd.org" also uses some version of gdnsd
• Logitech - gdnsd does the DNS for Logitech's SqueezeNetwork service

If you're not on this list and want to be, please drop a line! I know I've talked to people deploying gdnsd at various other companies in the past (and some have even provided patches!), mostly CDN providers, but I'm not going to assume you want to be listed here unless you've already mentioned it publicly elsewhere.