This must be one of Dave Winers favourites: Grazr is a DHTML based outline browser: You can link to OPML files (that again may link to OPML files). You can create virtual hierarchies of OPML files, RSS feeds and other Grazr outlines.
In any case Grazr is a slick little flash app that really connects the dots. This idea has bee around for years, but somehow no one ever took care of developing a simple tool for it. It was overdue.
By the way: OmniOutliner is an application pre-installed on many Macs that allows editing & exporting outlines as OPML.