Ironically, I discovered this cocktail at an antichow All Inclusive resort in Puerto Vallarta.
It is simply tequila and grapefruit soda on the rocks with a salted rim. I find Ting and President's Choice Splendido Grapefruit Frizzante both work well. In Mexico I think they were using a soda called Kas, but when I tried to drink that plain it had very little flavour (maybe a problem with that particular fountain?) so I am not sure.
Anyway, I love this cocktail, maybe even more than my longstanding favourite the margarita. It is bitter, sweet, sour and salty. Give it a try.
Any other salt-rim cocktails I should know about?