I got this from Allie Smith several years ago. When I had to dig around in my recipe binder to find it, I knew it was time to post it here to avoid future searches.
4 cloves garlic, chopped
1 jalapeno, chopped (if I don't have one I use an onion)
2 Tbsp. white vinegar
1 large handful cilantro, chopped
1 tsp. each kosher salt and pepper
juice of two limes (I do about a 1/3 cup of the bottled stuff)
juice of one orange (about 1/2 cup of refrigerated orange juice)
1/2 cup olive oil
Put everything in a large container with lid or gallon size baggie and shake well to combine. Add meat (we use a flank steak or 4 chicken breasts). Marinate 3-8 hours and grill. Slice into thin strips and serve with fajita fixings.