iPhone 3GS only: The Nearest Tube iPhone app uses the iPhone 3GS's newly-adequate camera to show the direction of the nearest metro station by adding directional arrows to a real-time "video" you take of your surroundings. Seriously, check this out.
AcrossAir, the makers of the app, claim it as one of the first "augmented reality" apps for the iPhone, meaning it adds digital signs to a real-time video. It only supports the London Underground for now, and is currently being approved by Apple, but we could definitely see this coming in handy for most any major city. Can we humbly request a version for Chicago, city of the miles-apart subway stations? We'll let you know when it's eventually approved and released into the App Store. [AcrossAir]