Google Earth allows for the creation of custom content by users. The easiest way is to create a kml or kmz file. kml and kmz files are fairly easy to create (or modify). It is just a mark-up language. If you can code html you can code kml files (they're not the same, but are similar). KML Tutorial. For those who don't want to delve into the code you can create these files within Google Earth itself. I mention kml amd kmz because this is the format you will save Google Earth files in if you want to make them available for others.
A kml file is simply a small text file with instructions for Google Earth to interpret. KMZ files are compressed archives of files. A KMZ file includes the KML file and any other files (like pictures) the KML refers to. I was playing around the other day and created a KMZ file from a nature walk (crosswinds marsh.kmz) my son and I took. I plotted our path (using my GPS) and took pictures along the way. I placed pins in Google Earth and edited their descriptions, to include the pictures. You can use html code in the description to change text style, include links to the web, bring in pictures, or even some web videos. I put the path and the pins in a folder in GE and saved the folder as a KMZ file.
I created the path using GPS Visualizer, a web application that converts GPS files to other formats, including KML or KMZ files. This allows me to use my GPS unit with the free version of Google Earth. Normally, you can only use a GPS unit with Google Earth Plus.
In the newest version of Google Earth Beta 4 they took some of the premium features from the paid version and brought them over to the free version, including the ability to create paths and polygons. So, if you don't have access to a GPS unit it would be easy enough to draw a nature walk and incorporate pictures from the walk as pins. Or have a student studying the Tundra biome outline the tundra and have pins marking points of interest with pictures and information. This would be a lot more fun than having the exact same information in a report or powerpoint presentation.
HTML Primer for Google Earth: This shows some very basic html code you can use in your placemarks within Google Earth. A quick search of the internet will yield a lot more.
The Rock Cycle - This is a work in-progress. As I complete more I'll be updating it. I will be doing this assignment with my ninth graders in January. The assignment is to create an illustrated tour through the rock cycle. The stops along the way must include photographs. The report is to be presented in Google Earth with pins added at the locations the photographs were taken.
Google Earth for Earth Science - Site created by Steve Kluge and Eric Fermann with lots of ideas on how to use Google Earth. It also includes an Intro guide to Google Earth that walks you through how to create Tours and Overlays in Google Earth.
SVS - Scientific Visualization Studio from NASA. Cool animations now available in Google Earth.
NASA Visible Earth - Great source of Earth Imagery. A great place to go for overlays in Google Earth if you want to get views of the same area from different times.
Google Earth Blog - Source of some great kmz files and good ideas for how to use Google Earth. My Favorites from the GE Blog are (I had to stop myself from just listing nearly everything from the Science Section of the blog):
Juicy Geography - I'm still delving into this site, but it includes teaching and lesson hints.
National Weather Service Enhanced Radar Images - The site will create a KML file for the selected weather radar which can be opened in Google Earth or NASA World Wind (I haven't tried World Wind with this yet). The data are real time and use the time slider that was added to Beta 4. You can hit play and watch or you can drag the slider back and forth. Each frame has to be loaded, so it can take a little time. You'll want to load it up prior to showing it to students. Some times it loads fast and other times it is fairly slow.
Teaching Geoscience with Google Earth - I just found this site, but from first glance it looks like there's lots of good stuff here.