One easy, quick way to make last night's pasta a little more interesting is to add a poached egg. When you break the yolk, it oozes deliciousness and coats the pasta. This works well for a pasta that doesn't have much sauce (tomato or cream) to begin with.
Another nice pasta trick that works well with a cut like spaghetti is to make a pasta frittata: just mix in a few eggs and bake until set, then cut into wedges. This makes a good lunch (served with a salad) or first course (served alone).