Read all about it on the Official Google Blog: Happy Birthday, Google Earth:

We got so excited around here about the first anniversary of Google Earth that we decided to celebrate a bit early. Beginning today, you can download a brand new version, Google Earth 4. Running on OS X? Feel the love. Prefer Linux? Ditto. Yes, we’re releasing simultaneously for PC, Mac (universal binary for full performance on both Intel and PowerPC based Macs) and for the first time ever, native support for popular Linux distributions. And we should say “salut,” “ciao,” “hallo,” and “hola” to our French, Italian, German and Spanish users, because Google Earth is now fully localized for those languages in addition to English. This includes a UI localized to French, Italian, German, and Spanish, as well as local search, driving directions, geo-coding, and unique local information layers for those countries.

And… better resolution, KML for Maps, geo-coding, Maps for Enterprise …