What's the best way to exercise and "get results" without equipment?

Many exercise equipments are expensive. Treadmills, weight lifts that focus on more than just the arms, stuff like that cost hundreds of dollars. I was wondering what is the most effective way to gain muscle without having to buy something. I mean, there's always sit-ups, push-ups, and walking/jogging. But what else can I do? I live near a track that's open to the community, if that makes any difference. Thanks.
 
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