Rex (Live at the Fillmore) is the thirteenth album by Keller Williams, recorded live on February 8, 2006 at the Fillmore Auditorium in Denver, Colorado. The collaborative show features Williams, Keith Moseley (of The String Cheese Incident) and Jeff Austin (of Yonder Mountain String Band) performing bluegrass versions of Grateful Dead songs. The group performed under the name Grateful Grass. The album is available only as a digital download only, and was released on April 30, 2008. Proceeds from the album go to the Rex Foundation.