There's a great article at Treehugger on saving money with your iPhone. Specifically, they explore four categories: automotive, travel, energy consumption and green shopping.
Applications like iGas, which helps you find cheap fueling stations, and GasHog, which measures fuel efficiency, are listed. However, what's interesting to me aren't applications.
PrimoSpot lets New Yorkers (Manhattan and Brooklyn only) find available parking spots, even identifying parking meters that are about to expire. That's pretty cool, and should save you some driving around.
SmartLinc lets you monitor your home's energy consumption and even remotely control lighting, heating and AC use, all from your iPhone. It's $1,400US to get set up, but definitely a useful service.
Many people still yearn for an Apple tablet. The iPhone is that tablet. Once you start to think of it as a computer with great telephone software, you'll begin to see that's exactly what it is.