The models I based this idea from are free, and the programs needed edit the 3D files are also free: Netfabb and MeshMixer.
The iStab is a great way to keep a cutting implement on hand at all times. This can be used as a weapon for self-defense for the next time you're in a knife-fight like Michael Jackson in Beat It, or harmlessly used for everyday actions like opening packaging or cutting items to size. Forget highly-contentious 3D printed firearms and flimsy iPhone knuckle dusters, the iStab has you covered for all your cutting needs.
Here's what I used to make my iStab: