Okay, this is not really a side dish, but we like it whenever we make any mexican dishes. I don't know how long it actually keeps in the fridge, because it never lasts more than two days at our house.
8 vine ripened tomatoes (or a mix of hot house and vine ripened ones)
1 white onion
1 bunch cilantro
2 limes, juiced
1/3 cup white vinegar
1 jalapeno (or spicier pepper if you so choose)
5 cloves garlic, minced
1/2 Tbsp cumin
1 tsp seasoned pepper
salt to taste
Chop it all up and stir together. If it sits for an hour before you serve it, the flavors mesh better, but it is darn good freshly chopped as well. You can also add orange or yellow bell peppers if you want more color.