A new version of ArcGIS Explorer desktop was released earlier this afternoon, marking the 12th public iteration of what is arguably the best and most popular GIS viewer on the planet (and free to boot!).
Explorer releases occur on an independent release schedule (different than ArcGIS Desktop and Server) and rather than go by versions Explorer is tagged by its build number. The goal here is to simply have everyone install the latest, and never mind the version. But the build number helps to differentiate between releases and the functionality delivered, and for those organizations that need to verify enterprise-wide installations it serves as a useful tag.
Every now and then I'm asked when the first release occurred, and I came across an old slide that I've updated to include the most recent releases (shown below).
You'll see that the very first release was at the tail end of 2006, with more frequent releases in the early going as the product rapidly evolved, then slowing down as Explorer matured.
For an overview of this latest release see the What's New in ArcGIS Explorer Desktop (build 1700) blog post.