So first, the only real one I know, where you can really play online, is from a couple of French and was made like a year ago : http://www.visunove.com/
I almost know those two programmers in real life, and they made this in one night, and leave it in Beta a few time after.
The VN list : http://www.visunove.com/vns.parcourir.html?c=popularite
One of the VN I took part in, on this website : http://www.visunove.com/vn.html?id=76
A remake of Bakemonogatari in VN : http://www.visunove.com/vn.html?id=36
Most of the VN posted here are try and not really interesting to play, mostly 'cause the interface isn't actually enough ergonomic (it's a good start but some things are lacking, you must choose sfx OR song for instance, or can't have more than one character at once, you have some bugs etc.)
The interface with your differents paths ("portions" of texts, you can link them etc.) : http://puu.sh/jQ3m.jpg
The real interface to edit and make your story : http://puu.sh/jQ7g.jpg
... and with the librairie of files you uploaded : http://puu.sh/jQ7Y.jpg
Sadly enough, it's a deserted project, we was somewhat enthousiastic about that, plenty of ideas, like a converter ren'py->visunove etc etc.
So, I was also wondering the current state of that thing on the web. I know a web way to play VN isn't easy, you must think of plenty of things, obsolescence etc.
But anyway, if you know something about this, you're something like welcomed~
