"Google Maps preview in mail" can be enabled from the Gmail Labs tab under Settings. Any time you receive an e-mail containing an address or a Google maps link, this feature will automatically create a preview of that location containing an interactive Google Map.
Currently the feature extracts only US addresses. However, Google is working on making addresses around the world work.
Also, whenever you paste a Google Maps link into the post box in Google Buzz, it will automatically fetch an image preview of that location.
What do you think? Is this feature extremely helpful to you? Let us know.