So what’s new in version 5?

Mars in 3D
This is the latest stop in our virtual tour of the galaxies, made possible by a collaboration with NASA. By selecting “Mars” from the toolbar in Google Earth, you can access a 3D map of the Red Planet featuring the latest high-resolution imagery, 3D terrain, and annotations showing landing sites and lots of other interesting features.

See global changes with decades of historical imagery
If you’ve ever wondered how your local area has changed throughout time, Google Earth now gives you access to the past. With a simple click, take a look at suburban sprawl, melting icecaps, coastal erosion and more.

Dive beneath the surface of the ocean
In the new ocean layer, you can plunge all the way to the floor of the sea, view exclusive content from partners like BBC and National Geographic as well as explore 3D shipwrecks like the Titanic.

Track and share your paths with others
Take placemarks a step further and record a free-form tour in Google Earth. Simply turn on the touring feature, press record and see the world. You can even add a soundtrack or narration to personalise the journey.

Sounds to me like it’s a good time to upgrade or try this one out. Feel free to let me know what you think, or share a “trip” with the rest of us…

Developers can now use the Earth as their canvas and apply their creative vision to a geographically rich, 3D environment, leveraging the same technologies Google use in the desktop Google Earth client. For consumers this means that they will probably soon see Google Earth in many more places around the web — perhaps even their favorite web site.

The Google Earth Browser Plugin is now live at code.google.com/apis/earth. Download it, learn from the online Developer Guide and samples, build something cool, and share it with the world!

Note! The plugins is currently only available for FF2, IE6 and IE7 running Windows.

The Official Intro

And another from gearthblog

Visit earth.google.com for more information.