Flash and AIR Debugger, Open Source
For my Flash projects, I've started using a debugger called MonsterDebugger which allows you to debug right from the browser, or if you're creating an AIR app, right from the app it self. This is great because the default Flash debugger doesn't allow you to do much if your app is embedded in a CMS (like Drupal or Joomla), or relies on some kind of server side language for input.