I use most of the things everyone else has already mentioned. Some other ingredients are peanuts, green pimento olives, green peppers, purple onion, grated cheddar, cottage cheese, leftover chicken, maybe some tuna. A little of this and a little of that!
When I absolutely feel like I have to put croutons on my salad, I cheat a little, and crunch up a couple saltines and sprinkle those on top after I put on the dressing. They only have about twelve calories each.