I've covered several different ways to manipulate the lowly pushup in previous posts. Many of these I'm sure that you've seen before. What you may have not seen (I certainly haven't seen it much) is ways to change up the pushup by manipulating your legs. All of these seem to have one common benefit: They are going to add challenge by forcing your core muscles to engage more. To what degree depends on which variation that you're doing.
The swiss ball pushp is the most common way to add challenge by destabilizing your legs. With your feet moving all over the place you're going to have to contract your legs and abs far more than what you're used to. I used to look at the swiss ball as a sissy girls fitness tool and so I wrote off these pushups. BIG MISTATKE! The added heigth really makes these hard. If that is too much for you, draw on your lessons from elevation manipulation and get your hands on some chairs. If you don't have access to a swiss ball, or desire less heigth, use whatever ball that you have that will hold the weight of your feet and legs.
A second way to change it up is to do what are usually called spiderman pushups. In this variation, you're going to lower yourself to the ground while bringing one of your knees to your elbow as you descend. From here you have two options: You could bring your leg back to the starting position and bring the other leg down and back on the next rep or you could leave it there and continue with your pushups. The former makes a great compound exercise that will challenge your obliques and hip flexors. The latter will be a powerful oblique conditioner. Both of these variations add challenge to your chest muscles because the weight of the leg will be supported not by the ground but by your chest muscle on that side. You can also do this one with your feet elevated as well. Either way, work both sides.
I haven't experimented with this one extensively but this is easily the most difficult version of the three. Find a wall or a door and walk your feet up the wall and do some pushups with your feet pushed against the wall/door. Make sure to keep your back straight. This is brutally intense because you have to it reasonably slow so you don't slip and you have to contract most of your lower, middle and upper body in order to stabilize yourself. The higher you go up the wall, the harder it gets.
I hope that I've been able to convince you not to overlook the basic pushup. There are many ways to spice it up and extract more usefullness out of it. I've only scratched the surface. It is a wonderfully versatile exercise, capable of doing many great things for your body.