1) Mighty Fine Burgers. There are several locations around Austin. Started by the same people who created Rudy's BBQ. Excellent burger, good shakes.
2) Hopdoddy's. Always rated #1 in Austin, but I've never been there. They always have a really long line.
3) Red's Porch is mainly a Tex Mex place, but I had a burger there once and it was pretty good.