Who Is Making Additional Material For Their Features?

I am not even talking about true transmedia work with developed story lines and expanded narratives; I am just wondering what examples are out there of additional material that has been used by filmmakers, mainstream and the indie DIY side both, to help bring audiences to the films.

Rainn Wilson tweeted about the shorts he did with Slash for The Rocker a few days ago, and I checked them out, but at that time, six months after the release less than 300 people had watched them on YouTube.
We have the videos s that Arin & Susan did for Four Eyed Monsters and set the bar for indie film promotion.  We have Judd Apatow's Knocked Up skits, and Wes Anderson's short for Darjeeling Express.  But what else is there?  Why isn't everyone doing it?  I would think that it is by now standard practice, but no.  It's not truly a money issue because there are lots of ways to do work on the cheap.
On Adventureland, we came up with a couple of short pieces that will soon debut on iTunes and elsewhere, but that was the first time that a studio "let" us do it.  I want to do it on every film now, and hopefully scripted well in advance.
Let us know what other examples you've found.