my favorite green salsas in austin are Guero's and South Congress Cafe. the Guero's green salsa has pretty good spice and a nice citrus-green apple flavor to it. the SCC green salsa has very good spice, similar to the spicy green salsa at Trudy's (which owns SCC). though it is not technically a salsa, the spicy hatch green chile sauce at Chuy's is also very good and potent for a green sauce.
my favorite salsa in austin, as of the moment, is the habanero salsa at Habanero restaurant on Oltorf. just ate a whole bowl of it for lunch, and my entire body is on fire. you have to special order it at the restaurant, but they'll give you plenty and a cup to take some home.