1 can tomatoes (diced will make a thicker salsa, whole will make a wetter one)
3 jalepenos or another pepper of your choice
Some form of garlic (you can use garlic salt, but really, use fresh if you can.)
Toss your peppers either in a hot pan or on the grill and cook till they're soft and blackened on all sides. The skin should be blistery. The broiler will also work.
Blend your tomatoes in the blender. I like smooth salsas, so I puree them. If you want more chunks, pull some of it out when it's at the chunky stage. Cut off the tops of your peppers, split them, and remove the ribs and seeds. If you're afraid of spicy flavors, start with blending one pepper into your tomatoes and work upwards. 3 peppers is what most people would call a medium salsa.
Add 3 cloves of garlic and enough salt to make the salsa taste a hair over salted (this will mild down as it cools.) Then, chill.
Texture or color: Diced avocado, diced tomato, diced onion, chopped cilantro, blending a tomatillo or some tomatillo salsa
Flavor: Lime juice, dry chile powder, crumbled dry oregano, black pepper, cinnamon, chipotle pepper in some form
Also, you can fry the salsa in a pan, which adds another flavor note.
What to do with it? You can spread this on fish and grill or broil it. Serve with chips. Use with nachos. Mix into chili. Serve as a topping for tortilla soup (at it's simplest, that could be chicken broth, shredded chicken, tomato, pepper, onion, and some baked crispy bits of tortillas.)
It should keep for about 5 days in the fridge, and you could probably freeze the base salsa sans any fresh additions to thaw later.