The Iron Cactus is the quintessential southwestern cuisine spot. They have many strong dishes, a good atmosphere, and arguably the best tequila menu in the entire country.
They have two locations, one located on 6th street and the other located in northwest Austin near the arboretum -- both are excellent, but different experiences. The rooftop patio at the downtown location is cooled off by jets of cold mist and provides an excellent opportunity to view the sights and sounds below, especially on a busy Saturday night.