It's kind of like an open space session but with a twist. I like it a lot and plan to experiment with it at one of my future London Knowledge Cafes.
Watch this 3 min video: How to Run a Lean Coffee This is how the process works for a small group of 3 or 4 people. But clearly, you could have several groups all running in parallel.
It was designed in 2009 by Jim Benson and Jeremy Lightsmith for Lean and Agile project management but could be used to discuss anything. You will find another description of the process here :What is Lean Coffee?