Goya Mojo marinade - go get a bottle today! We recently tired this stuff and it is great! It is a mixture of lemon juice, orange juice, onions and garlic.
We marinated chicken and flank steak in ziplock bags for at least 2 days (the bottle says the longer the better) and it was great on both. It gave the meat a nice tanginess and kept it tender and juicy. I think this would be great as a fajita marinade. We plan on buying it again as it makes for an easy dinner!