What are the undocumented features of Angular?

Angular 1.0.5 and 1.0.4:

• Scope has two useful undocumented properties:
  1. $parent - can be used as a hack for binding to a primitive property in a parent scope - example
  2. $$phase - can be used to determine if a $digest is already in progress - see this SO answer
• ng-switch has an undocumented change parameter that accepts an expression (example plnkr)
• select has an undocumented ng-change parameter that accepts an expression
• Attributes has an undocumented $observe method (although it is mentioned on the Directives page)
• $cacheFactory has two undocumented methods (example fiddle)
  1. get()
  2. info()
• the $location service has two undocumented events
  1. $locationChangeStart(evt, newUrl, oldUrl) (example)
  2. $locationChangeSuccess(evt, newUrl, oldUrl) (see also Angularjs - what's the difference between $locationChangeSuccess and $locationChangeStart?)
• use ng-value (undocumented) with input type="radio", see Disqus comment in API docs