There are already 11 certified Java EE 6 servers. It is actually amazing--Java EE 6 was released in 10 Dec, 2009. So we get almost every other month a new Java EE 6 server released. But: there are also servers available like Siwpas which are not certified, but are also working well (at least do not smoke after booting).
I smoke-tested some of them:
NEW workshop: Building HTML 5 Applications With Angular 2
Full week in December 2016: from Java EE 7 Bootstrap, Effective, Architectures over Testing to Microservices
On demand workshops: Java EE 7: Bootstrap, Effective, Testing and Microservices available for streaming.