The standard pushup is definitely one of the best exercises ever invented for upper and mid-body strength. You can hammer the shoulder structure, the chest, triceps and abs from the basic pushup. I don't want to make the pushup sound deficient in any manner but you can change the ease or difficulty just by elevating your legs and/or hands and get a much more complete workout, depending on what you're shooting for. All you need is something that will support your bodyweight. This can be cinder blocks, chairs, stools, benches, milk crates, etc.
Raising your feet above your upper body will place more of your weight on your upper body. So, as you raise your feet, you will make the pushup harder to perform. Subsequently, as you put your upper body above your feet, you will decrease the amount of weight you're pushing, making the exercise easier.
Let's start easy. Take your blocks/chairs/stools/benches (equal height! herein known as the elevators) and place them together. Place your hands on the elevators and perform a standard pushup. This variation is good for people who can't do a full-range, chest-to-floor standard pushup for reasons of weakness or injury. Do these until you can master the standard pushup.
Once you've got hands elevated and the standard pushup, now it's time to adding some challenge: The Atlas Pushup. We're going to elevate the hands again but instead of dipping only until the elevators, we're going to dip BETWEEN them. This adds more range to the exercise, rendering it more difficult. This pushup is a legendary chest-builder.
Once that becomes too easy, then it's time to elevate the feet above the upper body. If that still is too easy, you can do the Atlas Pushup with your feet elevated.
Of course, easy is kind of a fluid concept and I don't want to suggest that the other exercises don't have merit if one is harder than the other. You have to factor in how tired you may be after your day, if you're getting over an injury or some other minor physical impairments, or if you're trying to gain or lose weight. If you're trying to lose weight, you may want to go for a pushup that you can do in higher volume. If you're trying to bulk up, you may want to lean towards a higher intensity pushup. They all have their place, depending on your current fitness situation.