Welcome to WikiSpheres
A WikiSphere is the name for the environment a wiki gives you. The wikisphere is made up of syntax and GUI elements. When the syntax is unchanged from regular PmWiki, it is just a regular distribution.
Within this group a family of PmWiki derived distributions are presented designed to get the most out of the power that is PmWiki. And to get a good point to customize from. The basic PmWiki install after all is just that: basic. There are lots of features you'll want and you don't have to figure them all out alone.
WikiSpheres mean wikis that target the moving target that is PmWiki for you. There is less customization to do, less detail to worry about, which means more guarantees things will continue to work as desired.
One important consequence is that it will now also be possible to run PmWiki with a truely alternate syntax. Though you of course always could have customized your PmWiki to do the same, in practice you'd need quite a lot of stamina.
Ideally this means you can have one of the best wikis and still run your own rules. Alternate syntaxes in turn will make PmWiki better. As alternative wikispheres (made of syntax-rules and GUI elements) develop, PmWiki may one day opt for the most elegant design. In the mean time the current PmWiki wikisphere is a compromise with an installed user-base: it just can't be experimented with.
Pm is supportive of the concept: PmWiki was, after all, made for this to happen. It is entirely feasible to have PmWiki emulate most wikis out there, and this is the section you'd look for them.
However, if you're cautious about new syntaxes and want to be compatible to some degree with most other wikis, you should probably consider PmWiki itself, as it is in all likelyhood the most robust.
Status and support
As yet there are not yet any alternate wikispheres. That's the status. This is just getting started. Pm is working on a version Creole and there is a proposal to deviate from PmWiki, starting off with a (:linebreaks:) ON setting and develop a compatible wikisphere from there.
On the topic of support things are clear. Pm as the single person that defines PmWiki continues to do just that. That guarantees us the wiki core technology that is small and simple to install and extend. That's it. W7 wikis rely on a smaller user-base and (when of age) use separate mailing lists. Each W7 wiki targets and supports its own audience.