I was able to install iOS 7 beta 6 on my iPhone 5 without a developer account.
The trick mentioned here still works:http://www.iphonehacks.com/2013/06/users-able-to-install-ios-7-beta-without-developer-account.html
Here are the steps again:
Downloaded the correct .ipsw file for your iOS device
connect your phone to iTunes and (Alt/Option for mac, or Shift for Windows) on the Update button instead the restore button.
I was pleasantly surprised it still works.