Web navigation has gone through a handful of significant changes since the creation of the hyperlink -- from lists, to tabs to what appears to be a new user interface evolving out of tag data.  

The best example of this that I've seen is on a new site still in beta called Wikio.  This 5 minute screencast details what they've done (my first attempt at screencasting).

Looking back, it makes sense that this advancement would appear now.  People have been using keywords to describe content for a long time, but there haven't been obvious ways to leverage that data within a web site.  

Here are some of the major leaps in web site navigation interfaces:
  1. List view.  Lists of links to pages on the Internet made it possible to get from here to there and then on again to the next place.
  2. Left-hand column.  The invention of tables allowed us to break up pages into columns.  The left-hand side of the page turned into a natural location for lists of access points to more stuff.
  3. Search boxes and results pages.  The search box became a navigational layer on top of the list view.  It narrowed large lists to more manageable sizes.
  4. Tab-based navigation.  Left-hand columns quickly became overcrowded.  They were fine for people who were willing to work to find site-level links, but they were terrible in aiding discoverability.  Tabs were simple and impactful.
  5. Tag-based navigation.  This is a new layer that preempts the search box in a way.  The visual representation of it is a tag cloud, but the interaction is more like a pivot.
There's a ton of extensible value that comes from the relationships between items that share common tags.  And you can expose that value to people by using the tags to create better browsing experiences.  I'm hoping that we see more publishers building navigation for their sites this way. 

UPDATE: I showed he wrong version of Jon Udell's tag-driven search engine in this screencast.  He built one that allows you to dive in and jump out of a large collection of InfoWorld articles as you pivot on tags here: http://udell.infoworld.com:8005/.  He calls it the Metadata Explorer.