The key to a good salsa is to ensure that the tomatoes and onion and garlic are all chopped very finely. You want small little cubes. Look at the "Knife Skills" section of my post to find instructions on the best way to chop onions finely.
This neutral salsa can be adjusted to suit different cuisines: add jalapeno or habanero chillies to go with your Mexican food; swap the coriander (cilantro) for basil to use on Italian bruschetta; or add some fiery chilli flakes to serve with poppadoms in an Indian meal.