I have a couple of server that I want to play around with and my question is what flavor of OpenSolaris would you recommend? Nexenta, schilliX,MartUX,Belenix,Solaris Express,OpenSolaris Development Preview. ?

Solaris Express is the most up to date distribution. Perhaps Indiana (OpenSolaris development preview) will became the "reference implementation" in the future.

Far be it for me to argue with jlliagre re Solaris.
But ...
I had Solaris 10 in for a while - went to get some updates and I had to register. That gets up my nose. Same applies to Express. I will download the preview tonight and see if I like OpenSolaris - been thinking about it for a while. Used to be issues with grub - I'll see what falls apart on my test box.
I'm really more interested to see how the Solaris on zSeries port will develop, but might as well see what OpenSolaris looks like.

Get up your nose ? I'm learning English expressions on this forum but after all that's why I registered on LQ in the first time: to practice that funny/bizarre language that dominates the world.
Well, back to the point: What's wrong with registering ? You only need to do it once and as far as I know, security patches and Solaris 10 new releases are both free
There are no patches to SX, you just update or install the latest version and you are done.
I don't think there are changes yet with GRUB on Indiana. What were the issues you had with it ?
Got a mainframe in your garage ?

It goes against my view of FOSS.I seem to recall Sun insisting on you using their loader (prior to 10 maybe ...) I got Solaris 10 to chainload o.k. from my Linux system, but I deliberately built it first to ensure it didn't screw up my other systems. We will see what transpires with the so-called open system grub shipped with OpenSolaris.Not a zSeries, but I have access to a couple ...

I suspect there are legal or financial reasons behind that registration requirement. Solaris Express is not FOSS but built by Sun Microsystems from FOSS. It is more or less a beta release of the next commercial Solaris release. The fact Solaris is free (as in free beer) doesn't change that.
OTOH, Indiana, Belenix, Nexenta, marTux and Shillix are built by the community or individuals from the same FOSS so aren't bound to by these constraints.
Yes, until Solaris 10 update 1, a proprietary boot loader was used on x86 h/w.
I have no problem booting Linux with it. The only issue is you have to manually copy the Linux entries in Solaris menu.lst. Not that difficult.

Seen this ???.
And no, I not thinking of bidding.