I've had my iPhone for a little over a year now and don't know how I lived all this time without it!
Besides the 1,001 things it does, the way I use it specifically for real estate is as follows:
1. Internet access either through wi-fi or 3G networks.
2. Zillow makes an app for the iPhone. There are probably many others.
3. I have multiple e-mail accounts on my iPhone including gmail. Can view .pdf files as well.
4. I have a spreadsheet app that I uploaded my property analysis worksheet into.
5. Using the Google voice-recognition app, I can drive around, speak the address I'm looking for into the phone (instead of finger-fumbling), and Google gives me a map to the property.
6. Camera for taking photos of properties and things that need repair. I can then show the photo to a contractor or handyman and they can see what I'm talking about.
7. A notepad app for jotting down things, like items for a Home Depot run.
8. A voice-recorder app. I sometimes walk around a property and simply speak all my observations and notes to myself for later retrieval.
9. Calendar app with the ability to set alerts, e.g., "Meeting with Steve, 1:00 at Starbucks".
10. Finance calculator apps. Quick, what is the monthly payment on a $78,000 15-year loan at 5%?
11. Expense tracking software to log down mileage, receipts, etc.
12. A multi-function tool app that has a spirit level, unit converter, and dial caliper (handy for measuring small items like nuts and screws at the hardware store).
13. Checking the weather. Is this weekend a good one for an open house or repair call?
I have a few other apps that I haven't used for real estate, but I guess I could:
a. Aroundme (and Yelp) to find restaurants and gas stations in unfamiliar neighborhoods, as well as distances to nearby hospitals, supermarkets, and theaters when talking to prospective tenants.
b. A first aid app for that time that I'm sure to put a nail through my thumb! ;-)
c. A sound meter app for measuring or comparing the noise levels at certain properties (near a freeway?).