Definitely among my favorites. The title says spaghetti but fettuccine and linguine are quite OK. A handful of fresh green beans, cut into 1/4" pieces, is a fine addition.
Bacon. Use smoked bacon if possible (or prosciutto). Trim as much fat as you wish, then dice into pieces no larger than 1/4". Sauté bacon and onions together with canola or olive oil.
Tomatoes. Canned whole plum tomatoes are my first choice. Because irregular tomato pieces better cling to the pasta, either chop the tomatoes in the pan while they're cooking or first put them in a bowl and squish them into pieces with your hands.
Herbs. Use thyme or fresh basil. If thyme, fresh has the edge over dried. If basil, tear the fresh leaves and add them to the dish in the last minute or two. Optional: Include about 1/2 tsp fresh rosemary and/or oregano leaves, minced.