I found these for about $5 to $6 dollars at Ross.  They allow you to go a little deeper into your pushups.  I like to change up the width I'm holding them apart and even rotate them to get various muscles from various angles.  You can do pretty much any style of pushup that can be done free handed, the bars will just increase the difficulty level some and allow the resistance of your body weight to do more for you with every rep.  Good piece of equipment to have for a very small investment.