Don't forget el pastor on Riverside for the categorically best tacos al pastor in Austin. I am biased, though, because I am a chilango, and it is the closest to what I am used to.
on the south side, La Fuente on Circle S road is also fantastic.
See its threads like this that make me miss living in East Austin. Used to Live off of Angelina near the state cemetary. I loved that area, really quiet, nice people and you could hear tejano late on saturday evenings from a club way down on 6th. Not to mention the concerts from stubbs in the distance (well before the noise ordinance).
Alas, I am now in Sugarland. I hope for some authentic Austin Tex Mex when I get down to austin for the games this year.