A couple of threads in the forum touch on this topic, but no consensus has emerged yet.
A quick check of Google (http://tinyurl.com/allqv6g
) suggests that up to now
Web-based writing apps are mostly used for writing instruction.
So far, I've tried out SimpleNote, Outliner of Giants, and Google Docs.
SimpleNotes is great for creating quick notes/drafts to synch with Scrivener.
Outliner is rather cumbersome, but it does create outlines.
Google Docs is full-featured, robust, and has good export and collaboration features.
But it does not have Scrivener's ease of outlining, formatting, or compiling.
I should also mention EverNote and SpringPad.
In both, you may write RTF files in outline arrangement.
But export is limited, and the display of Web pages quite mixed.
So far, Google Docs seems the best alternative, but it's not Scrivener.