Alper Haytabay had created a subpages widget that was pretty much exactly what I was after (for another site I was working on). I found one flaw that prevented the widget from displaying nicely. In fact, the flaw didn’t appear to come from the widget but rather some other anomaly I could not determine the origin of.
The problem only occurs when the widget is displayed in it’s own sidebar. If there are no sub-pages to the current page the sidebar (which should be empty – and hence not displayed) appears. I could not seem to find a way to override this so I created a little addition to the existing code. If there are no subpages, display a small (user-defined) tagline instead. I made one other change to include this widget on all pages/posts so that if there are no subpages, the tagline will always be present.
I don’t use this widget on this site, but an example appears here
The widget (my version) can be found here: subpages-widget