Is 'Just Sitting' the same as zazen? In the Triratna Buddhist Community we do a practice that we call Just Sitting. It's described as


  Just Sitting is a space of non-action in which anything can emerge.
  Often the fruit of the previous practice only emerges when you stop
  ‘doing’ it. And Just Sitting is the non-doing space in which that may
  (or may not) happen. Just Sitting also allows assimilation of what has
  just been done, and provides the necessary counterpoise to activity
  and effort. Just Sitting is a matter of simply ‘being’ with whatever
  happens in awareness, without attaching to it or rejecting it.


Link to quote is here. 

I've always wondered if this is something that we've just kind of invented or adapted or is it common to other Buddhist traditions? Is it the same as zazen practice perhaps?
The term Zazen (Chinese 坐禪, zuò chán) just means seated meditation. It doesn't itself specify what kind of meditation is being practiced.

Just Sitting refers to Shikantaza, which is the type of meditation taught by Dōgen Zenji, the founder of the Sōtō school of Japanese Zen. He famously explained it in the Fukanzazengi saying :


  Once you have adjusted your posture, take a breath and exhale fully,
  rock your body right and left, and settle into steady, immovable
  sitting. Think of not thinking. Not thinking-what kind of thinking is
  that? Nonthinking. This is the essential art of zazen.


There are similar practices in other schools of Zen in China and Korea, but it is less prominent as Koan practice is more common.