Windows only: The freeware Google Earth plug-in integrates Google Earth with your web browser, embedding the 3-D glory of Earth in Firefox or Internet Explorer. The results, which you can see in the video above, are impressive. There isn't a ton of Google Earth plug-in integration on the web yet, but check out the samples to understand the possibilities, like switching between Google Maps and Earth on-the-fly. The Google Earth plug-in is Windows only, supports Firefox and Internet Explorer.
Google Earth API [Google Code via Google Blogoscoped]