Currently (both for the past 5+ years):

• Go (golang.org)
  • standard library (golang.org/pkg/*, http, http2, etc)
  • continuous build/test system (build.golang.org)
• Camlistore (camlistore.org)

Previously:

• android.os.StrictMode
• LiveJournal
• memcached
• OpenID
• djabberd, perlbal, gearman, ...