Here's a fantastic pushup variation I've found myself using with increasing frequency in my own training, along with programming them for some of the athletes in SAPT who have progressed appropriately.

What is it: Pushup Bodysaw Series. You can choose one three options shown in the video below:

Why I like it

Ummm, 'cuz it looks completely awesome! While I have yet to confirm this, I'm pretty sure Batman used these in his preparation to battle it out with Bane, the verifiable genius and muscle-bound brawler we'll experience during The Dark Knight Rises this Summer. (Yes I'll be seeing this opening night in IMAX)

Sorry, I got distracted....let me try this again.

Why I like the pushup bodysaw series

You can do it virtually anywhere, without needing any crazy special equipment!
It's a pushup variation. When it comes to "bang for your buck" exercises, few can top the pushup. I rank them right up there with deadlifts, chinups, and farmers carries for best return for your investment.
It really increases the difficulty of the standard pushup by adding a dynamic component for the hands+scapula, providing an "anti-extension" challenge for the core, and decreasing the mechanical advantage via base of support shifting.
You get a bit of scapular rotation work with the arm that's sliding up.
The spiderman bodysaw (third in the series) increases the difficulty even further by taking away a base of support, along with giving you a bit of hip mobility as your leg goes into flexion and abduction at the top.

How to do it

Grab one or two furniture sliders, these are crazy cheap and can be found at nearly any hardware or Bed, Bath, & Beyond-related store. If you insist, you can omit the sliders and simply use a towels (if you have a slick surface to use, like a wood or tile floor), or paper plates on carpet.
Squeeze your glutes HARD (as if you could crack a walnut between your buttcheeks) and brace your abdominals as if the world is about to end. This will keep your hips and low back from "sagging" throughout the movement.
Keep your neck in neutral by resisting the urge to let your head drop, "reaching" for the ground. This is probably the most common error I see people unaware of.
Perform 4-10 reps per side.
Pretty self-explanatory from here, just watch the video and try to avoid falling on your face.

Depending on where you're at, I would NOT begin by trying to slide all the way out. Stay within a range of motion that you can keep good form in, and progress from there (most of you will have to watch out for the hips sagging to the ground). If you haven't already, master the perfect pushup before moving to one of the pushup bodysaw variations.