The site above is the beginings of an official "HCM Integration collaboration site" we're trying to get kicked off from within Oracle - this is currently by "invitation only".
The one's on the left are hosted under blogging guidelines and open to everyone.
The "Single Sign On" website on the left is the most built out & useful at this point. "Fusion Integration" & "Upcoming Presentations" are also worth checking out...
Also be sure to bookmark the following launch off page for Fusion Apps in My Oracle Support - Fusion Apps Product Information Center