Twitter has supported expanded tweet view for a couple of web services already. You can view photos and videos from services like TwitPic and YouTube.
They have now expanded support for expanded tweet with support for a bunch of partner sites.
Twitter would showcase preview headlines, excerpts and images. In some cases, you might even get to see the Twitter account of the author of that article.
Some of the newly supported sites are:
The Wall Street Journal
MSNBC’s Breaking News
The New York Times
The San Francisco Chronicle
Der Spiegel Online
You can of course also use the browser plugin from Embedly to add support for almost everything out there!