From Em Software:
Em DocsFlow brings the collaborative editing power of Google Docs to InDesign, providing dynamic linking from any InDesign story to a target Google Docs document, along with the almost magical ability to re-weave any Google Docs document changes into the linked story, on update. The breakthrough concept embodied by DocsFlow is to marry freely-available and popular web-based editing tools (Google Docs for now) to InDesign, giving you a zero-cost per-seat editorial workflow solution that is extraordinarily easy to manage. And using DocsFlow means you need no special resources to build, maintain or learn the editorial side (which is just Google Docs). Further, DocsFlow builds on the built-in InDesign story linking machinery to avoid any learning curve on the design side.
With DocsFlow, both the InDesign story and the Google Docs (GDocs, below) document can be edited independently, and the links palette will show you the status of both at any point. When the GDocs document is edited, you'll see the link's status change to "modified" (showing a yellow alert icon), and you can update the story from GDocs with a double-click on the icon. Any changes on the GDocs side are merged into the the story, even if the latter has been changed. After each update, you can open the story editor with change marks showing, to see who changed what. Most of the text-level formatting in the GDocs document will import naturally, and GDocs block-level styles are mapped automatically to InDesign styles. GDocs comments import as InDesign notes.