If you're subscribed to receive email from certain senders, the messages you receive from them will be enhanced with an interactive gadget that has up-to-date content from their website (you'll also see an icon in your inbox identifying these messages).
For example, if you receive a Pregnancy Bulletin newsletter from Babycenter, you'll be able to view up-to-date content, including the baby name of the day, and browse though the current top 100 baby names within the message. Aside from the convenience of being able to interact with certain websites from inside Gmail, the branded content will help identify that your messages are legitimate and not spoofed (we'll only show branded content when the sender authenticates their mail). We're currently testing this with a small number of senders and will decide whether to make it widely available based on user and partner feedback.
One of these partners is Netflix. "A Netflix email showed up in my Gmail inbox today, and it looked different than I had ever seen- a little Netflix logo showed up right in the inbox view, and when I clicked on it, there was a whole fancy pane below the email containing movie recommendations I could add directly to my queue," noticed Dan McGee.
MG Siegler was surprised to see a favicon next to a Netflix message. "I loaded up Gmail this morning expecting to see the usual assault on my inbox, when something new caught my eye. Apparently, Google has started inserting favicons, the little icons that many browsers put next to a website's URL or bookmark, next to messages. So far, I'm only seeing it for emails from Netflix."
It's not clear whether Google has a financial agreement with these partners or if the features will be available as part of an open API. Many of the experimental features from Gmail Labs enhance messages with previews and icons (YouTube previews in mail, Google Voice player in mail, Authentication icon for verified senders) and Gmail is an OpenSocial container, so a Gmail gadget API for messages is not far-fetched.