Not quite the admin GUI you're looking for though; it's more for lowering the barrier to entry.
NetObjects Fusion had some interesting features and ideas. My first non-intern job was with a company that did e-Learning materials. Our products were basically self-contained websites, with video content and scripted subtitled text, distributed on CDs. This was 2002 or so, so it was super bleeding edge then.
Our entire infrastructure relied on the extensive scriptability and plugability of NetObjects Fusion. We had an entire authoring system and video production pipeline that would allow NetObjects to generate pages for the videos with templates, all things needed included. All you had to do was drag the video player into the page and set the right property, and our build system would handle everything else.
The process was so automated on the technical side that it allowed the task of actually assembling the sites themselves to be farmed out to University students with little technical training. Although, speaking as someone who was initially hired as one of those "production workers," we were paid very well relative to other traditional college jobs and the work, while sometimes tedious, was often interesting and fun. I got to learn so many random things - how tires are made, how the propellant for the Space Shuttle's SRBs was mixed, etc.
Even now, I can still, a bit fondly, remember the hierarchical layout of NetObjects, with it's little yellow and black shields representing pages.
It was also a great company to work for. The owner was a business professor full time and this was his side project. We were always small, and him and his wife always took an interest in all the students that worked for them. Getting a home-cooked meal and a night of poker once a week was a nice perk for a poor college student.
The cool thing, though, was there was no pressure to stay. They knew we were students, they knew we were going to graduate eventually and take jobs elsewhere. It was a mutually beneficial scenario where they got relatively cheap labor (at the time), and we got good experience before heading out into the world.
I ended up staying on there for another 3 months or so after I graduated before taking my first full-time programming job. A big part of me landing that first post-college job was due to experienced I gained working in that environment. First as a lowly production worker, then advancing to graphic design, administering the network and programming plugins and scripting for NetObjects.
So I suppose my career would probably look very different without NetObjects Fusion.
Edit: was a browser included on the CDs? Today, one could bundle a portable-apps version of Firefox, increasing the likelihood of the content remaining readable, without needing external plugins for video.
EDIT: It doesn look like it's available for linux though...