Mexican

I'm not sure if the origins of this system are Mexican, my exposure to it has been through playing youth bridge against Australians many of whom are avid fans. The system revolves around the 2♣ opening which shows 18-19 balanced. Seems a little odd at first but when you realise that after opening one of a suit you either have a weak NT or an unbalanced hand there's all sorts of niceties that appear. Here's the opening structure:

1♣	11+ 2+ clubs
1♦	11+ 4+ diamonds
1♥	11+ 5+ hearts
1♠	11+ 5+ spades
1NT	15 - 17 balanced
2♣	18 - 19 balanced
2♦	23+ GF
2♥	6 - 10 6 card
2♠	6 - 10 6 card
2NT	20 - 22 balanced

So it's standard with the weak or multi 2♦ replaced by a Mexican 2♣. Over 2♣ obviously pass with long clubs and few points, 2♦ through 3♣ are all transfers, 3♦ is stayman and the rest you can use your imagination.

The cool thing that happens is when you hold: void Axxx AKQxx Axxx and the auction goes 1♦ 1♥ you can rebid 2NT which is game forcing with either solid diamonds or 4 card hearts. Now you've set up a game force at the 3 level and can start to investigate the possibilities of 6 or 7.
It also solves the problem that with a hand like ♠Kxx ♥AQxxxx ♦AQx ♣x, Where you might normally be torn between rebidding 2♥ and 3♥ you can open a heart and over partners 1♠ rebid 3♥ safe in the knowledge that partner won't expect the world because you didn't bid 2NT.

Variants:
I once played this substituting 2♦ for a "bad weak two in a major". For those not clear on the specific definition of bad in play here we're talking ♠Txxxx ♥Qxx ♦Kxxx ♣x
Partner should probably pass with four or more diamonds and a short major because they haven't doubled yet it might be a better fit. We were playing pairs and figured we wouldn't really miss having a game force opening over a couple of sets and being in the auction lots is always profitable. It's certainly different playing 3NT as 23+ any.