It's not open source, but there's a free tier and lots of nice features built in like video recording, screenshot comparison and a GUI editor.
If anyone is planning to write PhantomJS scripts directly, I strongly recommend checking out CasperJS: http://casperjs.org/ -- It gives you a really nice layer on top that makes Phantom much more wieldy.
Edit: Here's a very simple example http://searchproxy.automate.ly/
It would be interesting to see what type of customers or use cases your solution might be of use.
Will have a look. PhantomJS development seems stagnate a bit. Try to contribute to it myself.
Splash Web UI is quite similar to PhantomJS Pad UI - by default it shows a CodeMirror text editor (with a simple autocompletion) and a screenshot of the webpage. Also, by default it shows information about requests/responses (like a "Network" tab in Firefox/Chrome development console) and a textarea with rendered html.
PhantomJS Pad UI ideas are good; tabs for images, an editor window which is open by default and a field with console logs make a lot of sense.
The concurrency model in Splash scripts is not the same as in Phantom JS scripts: Splash allows to use loops/ifs/functions in async code without recursion tricks or things like "casper.thenOpen". See http://splash.readthedocs.org/en/latest/scripting-tutorial.h... for more info.
We haven't done a public announcement, and Splash is not yet feature complete, but it is used in production for more than a year, there are hundreds of unit tests and it is in active (though a bit sporadic) development - so give it a try if you want something different :)
Installation is just "docker pull scrapinghub/splash" - see http://splash.readthedocs.org/en/latest/install.html.