What functionality of iOS can be accessed from the lock screen? I know that in iOS, several parts of the iDevice's functionality can be accessed from the lock screen. This is useful when you don't want to fumble with a password.

But now I'm starting to wonder: what else can be accessed from the lock screen? I know these:


Three-home-button tap to invert screen (when Universal Access is enabled)
Double tap to play music


What else can be accessed from the iOS lock screen?

EDIT:

As Daniel L mentioned, 


  "It shouldn't produce a bunch of partial answers; a good answer will
  include a canonical listing (with sources) of what can be accessed
  from the iOS lock screen. There is a right answer to this question."


Please, make your answer a canonical listing (with sources) of what can be accessed from the iOS lock screen.
I'm including the ones mentioned in your answer in the name of having a complete listing in one place.


Toggle music controls (Play/Pause, Previous, Next, Volume) by double tapping the home button.
Take pictures and videos by sliding the camera button. 
Invoke accessibility features by triple clicking the home button. Configurable in Settings-> General-> Accessibility-> Triple Click Home. You can configure toggling VoiceOver, toggling White on Black, toggling Zoom, or asking which to do.
Make phone calls by tapping the Emergency Call button and dialing a number. iPhones only.
View snippets of emails, text messages, and other notifications by looking at their  notification preview. Configurable in Settings-> Notifications-> App.
Activate Siri to make calls, send text messages, send emails, get directions, check calendars, etc. by holding down the home button. Configurable in Settings-> General-> Passcode Lock. iPhone 4S only.
Take a screenshot by pressing the home button and sleep/wake button at the same time.
Power the device off by holding down the sleep/wake button.