Because it is so easy to add 3D to existing map mashups, this list is going to grow very quickly, but for today it is still worth pointing out some of the early adopters I’ve become aware of, so that you can go test the plugin.

• Housing Maps: One of the early original mashups, Housing Maps, now has an Earth button (though it would be great if Mac users didn’t get to see it. This goes for many of the sites in this post.) (via Trufflehoney)
• walkhighlands.co.uk: In the comments, Paul Webster of walkhighlands.co.uk writes in with an interesting discovery:
  

  Even better news is that the Earth plugin/ API fully supports kml network links with regions, unlike the Google Maps API.

  This means you can run huge datasets using just kml, as the Earth plugin will load more kml files on the fly automatically if you have the regions set up to load when on screen and zoomed appropriately. We’re working on making our site navigable using the new plugin – see here.

• Google Earth Hacks: Mickey Mellen writes:
  

  A page is up for every file on Google Earth Hacks, but many still have issues. When viewing any of our files, there is a link that says “New! GE plug-in” in the “Get File” section. Here is an example of a functioning one.

  More info by Mickey here.

• GolfNation: Mickey Mellen writes:
  

  Here’s another one I’ve converted – GolfNation.

  It has Google Earth content for every course. Some items are simple placemarks and some are detailed layouts of each hole. All of them now offer the option to view details using the plug-in. Here is a sample page.

• Travelpod: Mashable points out that travel blogging site Travelpod has gone 3D.
• See Bournemouth: Mapperz sends us to See Bournemouth.
• The Glacier Express: Google Maps Mania sends us to The Glacier Express.

Web applications that delve deep into the plugin’s extended API will take a while longer… but when these show up, I’ll try to post them.