Half-Life 2's relative simplicity and versatility has encouraged many hardware and interface developers to make it one of the first stops for testing out their work. Milan Toth has recently demonstrated his iPhone application Remotion to raise some hell in City 17.
Using it seems simple and intuitive. If you have an iPhone, this app is only 99 cents so if you give it a go let us know how it feels.
All I need now is an iPhone, Big Screen mode, and way to strap my phone to a Nerf gun.