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:
Online workshop: Java EE 7 Bootstrap