Smart phones are really great tools. They are especially great for private investigators. I finally broke down and got an Android phone, and I have used it like crazy. You can take notes, keep track of your mileage, integrate photos with map locations, browse the web, and you can use skip trace services on the spot.
I use the maps and latitude feature on my phone all the time. I can track movements from surveillance and spot my exact location right away. I have a Tom Tom which I love, but this app helps me go back and follow up much easier.
If you do civil process service there is an app that is great for that too. You can get a good app from http://www.truthinservice.com for about $80. I contract with a company and do process service and they have their own app. It seems to work pretty well.
I also use Droid Scan a lot for copying documents. All you need to do is have decent lighting and use the phone's camera to capture the document. The program does a good job of automatically cropping the edges of the document. It works really well and you can upload scanned documents to Google docs for later use. The OCR feature is pretty good if you need to make reports using information out of the documents.
It takes some practice and time to get really good at all of these apps, but there are many free ones that can get you going and help you with your work. I went a route that many may or may not want to go - I did the straight talk thing with the $45/month unlimited talk, text, and data - so far I have been happy with the phone, features and service.
Whatever you do out there - be safe!