Place all ingredients in a blender and blend for 5-10 seconds or until cilantro is pureed and integrated into the sour cream. Don't pulse too long or it will be runny.
Add more salt and lime juice to taste.
Notes
This sauce is versatile. I generally use it to cool down a spicy taco but if you like heat then add jalapeno. Some other ideas are ground cumin, hot sauce, lemon juice (to substitute for lime juice), coriander, oregano, or garlic.
If you blend the cilantro sour cream mixture for too long, it will denature the sour cream and be to runny. It will still be delicious so don't worry to much about it but if you want to thicken it up, remove some of the liquid mixture and manually stir in additional sour cream. Add salt to taste.
Don't have sour cream? Try yogurt. Don't have lime juice? Try lemon juice or a touch of vinegar.
One of my favorite taco recipes to pair it with is Chicken Enchilada Tacos with Lime Cabbage Slaw.