I caved in to the urge and got an iPhone 3G . . . and have been amazed. It's not 'just' a phone, it's a tiny computer and a GPS. It instantly replaced five 'gadgets' I usually have with me: phone, iPod, computer, camera, and GPS. An unexpected bonus is that I have found it a perfect journaling tool.
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What bird was that? The other day we drove down to a pond south of us, and were astonished to find hundreds of birds enjoying the bounty of water in the desert. I didn't have my full field guide set with me, but I was able to confirm Common Ground Dove calling with National Geographic's Handheld Birds app - most every entry includes sound as well as images and data.
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I have a real ineptitude for remembering astronomy. My all-time-favorite app thus far is StarWalk. Like having a mini planetarium and personal starguide in your pocket. It's difficult to describe just how cool this app is - you can zoom in and out, scroll 360 degrees around the dome of the sky, including what's below the horizon . . . click on a planet or star or cluster and get full data on it . . . and search for the name of a celestial object and it will show you where it is - all in exact relation to your current location. So when I note in my journal the arresting moonscape-and-planet I saw at 4 am, I can look up exactly which planet.
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I always note the times of sun and moon rising and setting, as well as the moon phase. This great app, called Sun and Moon, always keeps it at my fingertips.
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With the GPS and two very useful apps, I am always able to follow our route on topographical maps in the U.S., or using Google maps elsewhere. Topo Maps has a full set of USGS maps for download, and GPS MotionX turns the iPhone GPS into a fully functional track and waypoint tool and then some. I no longer need my Garmin, and furthermore it's much easier to use, download maps, and export and share data.
Finally, I am always using the built-in iPhone camera to take snapshots of things I want to sketch, like beetles or a nice sunset. And I can use All-in Notes to take quick voice memos or combined photo / note / memos and email them to my main computer.
All the apps are available at the App store; just use the Search function to find.