They are called tethering apps and there are currently none on the App store. Apple tends not to allow that because the carriers frown on it heavily. There have been one or two in the past that slipped through because they were named something unrelated to their actual use, but Apple caught them and removed them.
You are posting this in the jailbreak forum so I have to assume your iPhone is jailbroken. If so, you have options. MyWi, TetherMe and PDANet.
I use TetherMe. It seemed, at the time I looked, to be the one tethering app that was the least discovered. I think only PDANet is free.
Install and use at your own risk. The carriers have ways of detecting tethering and if you don't have a plan you'll get charged.
If you are not jailbroken then your options are to jailbreak and install a tethering tweak or pay Sprint for a 1GB, 2GB or 10GB hotspot feature.