A long, long time ago Rise of the Triad was meant to serve as a sequel to Wolfenstein 3D. It was developed by Apogee Software, and the code became generally available to the public. It is said that the game has been ported to almost every platform, and I believe iPhone is no exception.
The video shown below is only a very early version of the game, and the controls are susceptible to change. In the original game, you were able to pick one of five characters that each had their own abilities. Stay tuned folks, this may be a winner. The developers for the iPhone port also happen to be the developers for Tyrian, another old shoot-em-up ported using open source code.