Stir together lime juice and agave nectar; add to a blender, along with pineapple juice, pineapple, triple sec, tequila, and ice. Blend until smooth and frothy.
In a small shallow bowl or saucer, combine chili powder, sugar, and salt. Rub a lime wedge around the rims of four glasses, then dip glasses in spice mix to coat. Divide drink among glasses and serve with lime wedges.