I had a honor to take part of the discussions at the JCrete unconference.
The event is invite only, and this makes sure that the crowd that gets together has a very good motivation to collaborate on ideas, discuss various topics - all the attendees contribute to the event, everyone is a speaker!
I have attended many discussions in the official part - at the scheduled sessions. But there was also "unofficial part" - the discussions were happening everywhere: in the car while driving to Falassarna, at the beach after lunch, at the dinner. So the density of useful information and ideas you get at this event is very high!
I have attended the discussions about Gralde, developer tooling, profilers, JIT & JITWatch, Java 8 lambdas, sun.misc.Unsafe, Asciidoctor, hardware performance counters, GC- and lock-free programming, and maybe some more. There's a wiki page that includes notes for the various sessions, contributed by the attendees.
The dates for JCrete 2015 are already announced, so if you like geeky discussions, Mediterranean sea and olive oil - I definitely recommend to attend the event.