If you're looking for al pastor tacos like they make in Mexico this is the place to go. I've tried several different restaurant's versions of the dish and they offer the best one. It's not the cheapest place for Mexican food but well worth the investment. It is located in the small shopping center next to the Kaleidoscope in Mission Viejo.
I really like Taqueria de Anda. I either get the tacos al pastor or burrito al pastor. They have locations in Placentia, Fullerton, Orange, and Anaheim. I prefer the location in Orange, on Tustin Ave one block south of Chapman. They have a "condiment" bar: red and green sauces, limes, onions, cilantro, and Jalapenos (fresh, not pickled). The menu has a price for refils but mine are always free.
If you are down by the beach, give Taco Surf a try in Surfside (Between Huntington Beach and Seal Beach). From Seal Beach Blvd, it is just over the bridge as you cross into Surfside on the right hand side.
I don't know about their whole menu, but every day during the week between 2 - 6 pm, you'll get $1 tacos and $2 drafts served by friendly folks in a cantina atmosphere. I try to make it down there every week.