There are hundreds of "green" iPhone apps. Many of them are junk. Some of them actually can help you live more sustainably. Here are a few I think are cool.
Dare to Share:
"Find a walkable place to live." Of course it is good for that--enter any address and see how convenient it is for walking to the things you need. Great for house- or apartment-hunting. It can use your current location, or you can enter one. My favorite use is to enter the addresses of companies that say they are "green" and see whether their offices are in walkable neighborhoods. Usually they are not.
When purchasing or ordering fish and seafood you don't want to support harmful harvesting or environmentally destructive practices, do you? With this app you can either enter a specific fish or seafood, or browse the guides. Also has crowdsourcing feature that lets you add restaurants that serve sustainable seafood, etc. Based on the Monterey Bay Aquarium Seafood Watch program.
If you are green you are using public transit, right? Nextbus is fantastic, where it is available. It depends on your local bus company or transit agency participating--some do and some don't. There are many apps that use Nextbus-like technology to help you arrive at the bus/tram/rail stop just before the bus/tram/train does. Never miss the bus again! Explore the App Store for apps that apply in your town. Search for "nextbus". In the SF Bay Area try Transporter.
You may not have an EV or PHEV (yet), but PlugShare is ready when you are. It's "a community-powered electric vehicle charging network that includes an
up-to-date listing of all public charging stations compatible with the
newest generation of electric vehicles like the Nissan LEAF and Chevy
Volt." The coolest feature for non-EV-owners (EV-nonowners?) is that you can list your outlet as a shared resource for EV drivers who need a charge. This vastly increases the availability of charging and helps keep EV drivers from getting stranded.
You don't want to get into arguments with climate skeptics--You can't win, and it is just too discouraging. But if you ever want that warm glow of self-satisfaction that comes from having those arguments literally at your fingertips you can get this comforting app. The app summarizes peer-reviewed climate science and helps you learn about what the science says, even if you can never use that information to convert climate skeptics (they can't be convinced by facts).
The Google Maps app for the iPhone doesn't offer the bicycling route technology that the web-based Google Maps has, unfortunately. And I can't find any other free cycling apps that I'd recommend. Any readers know of any?
Find nearby places to recycle all kinds of stuff. Old car battery weighing you down? Busted NES? Mushrooming cloud of plastic grocery bags? Moldy mattress? Mountain of Styrofoam™ packing peanuts? Tangle of wire clothes hangers? This app uses your location to query Earth911.com's database and tell you where you can get rid of them where they will be properly recycled. (Some places charge to take some things off your hands, but phone numbers are provided.)
Any other suggestions?
All of these are free. I am sure there are some nice apps that cost something, but you will have to check those out yourselves.
Many of these apps are available for other platforms too, but I tried them on the iPhone.
Seafood Watch image from the Seafood Watch site.
PlugShare image from this GigaOM article.
This is also crossposted to the SAP Community Network.