For campus/strip, Gus's is legendary for the food and nostalgia, but as far as hanging out for a while, you'd probably want to try Copper Cellar (Cut from the same cloth as Calhouns), Stefano's, down the road, has great pizza.
For downtown, I second the Grill and Brewery. Also heard this new place, Chivo, is really good (food, maybe not atmosphere).
Market Square has some places too.