At Eat Street's Pancho Villa, the margaritas may come from a mix, but they're served in an atmosphere of authentic ease and celebration. The icy, pale verde elixir flows in cheap abundance here, where it's served in green, cactus-stemmed glasses with dense flakes of salt clinging to the sombrero-wide rims. Informal, busy, and noisy, the restaurant's two well-used rooms play host to a constant reconfiguration of tables. Birthday parties bump elbows with after-work happy hours and casual weeknight meet-ups. The refreshing, sweet-and-sour margaritas are reasonably priced at $4.50 apiece, and they're two-for-one 'round the clock Monday through Friday. When accompanied by at least five friends, birthday boys and girls may drink up to six margaritas on the house, guaranteeing a blotto cumpleaños and a frugal fiesta for all.