This is a great barbeque marinade for sprucing up chicken and pork or would work well with fish and shellfish.  
Though not purely Mexican in origin (it's on my list of places to go!) it hopefully tastes a little authentic.  I did recently learn a few tips from a Mexican friend though; when making guacamole, sear your chilies/jalapenos first in the frypan for 5 - 8 minutes, so they get that lovely smoky taste, then chop up finely!

Mexican -ish Marinade
In a mixing bowl, mix together the following:
Zest and juice of one lemon
Juice of one lime
1 jalapeno chopped finely or half tin of Trader Joe's chopped chillies
Large handful of chopped coriander (cilantro)
1 tspn ground cumin
1 tspn chili flakes
ground salt and pepper

Spread over meat and leave to marinate overnight (or 2 hours for fish).