The PlayOn Media Server as I understand is not a channel in itself, but allows the running of scripts or channels to be installed through the PlayOn service.
The official list of PlayOn channels can be viewed here, but there are many, many lists of supposedly valid scripts / plugins for the PlayOn server.
To name just a few:
There is a good YouTube video about installing these plugins through your PlayOn media server.
I have not invested a lot of time verifying which scripts are functional and are not, but that is a topic we are hoping to touch on more in the PlayOn Plugin category of the Google+ community.
One member of the community has shared that her favorite scripts are "Solar Movie" and "Project TV."
Official Website: http://www.playon.tv/playon
Paid or Free: Free
Activation Required: No
Channel Code: PlayOn
List editor Google+ profile
Private website not affiliated with Roku.com
Roku is a trademark of Roku Inc.