Summer Glau wears this dress in the climactic fight scenes of Serenity at the end of the movie , as well as in promotional photos. This green dress is from the Donna Karan New York (DKNY) Spring 2004 Ready-To-Wear collection.
Here are promo shots and screencaps featuring Summer Glau wearing that dress :
For the record, this lovely dress by DKNY has been used in two different promo shots. It's likely not the same exact dress, as both were likely purchased in a store. The first promo shot was for the DVD cover of Dead Like Me season 2 (Costume seen on Ellen Muth as Georgia Lass, on the left on the photo), and the second promo shot was from Serenity.