The iPhone app that has proved to be the most helpful to me in my business so far is Jobs.  With this app, I can set up a job, link it to a client, set the hourly rate, make notes about the work session, start the timer, and Jobs takes care of tracking my time for billing.   When you stop the timer and restart later, Jobs will create a new work session; this is really helpful to me since I don't want all of my work sessions for a project lumped together.   Many times I'm working on a different task with each session, so this feature helps me analyze work efficiency and accuracy of quotes.

Some other apps that I literally use every day include weather, To Do's, and Pandora.  I've gotten hooked on the convenience of Pandora and the ability to listen to my favorite artists while also running across new artists.

Two other apps that I use a lot are for Facebook and Twitter.  These two apps allow me to make updates to my Facebook and Twitter accounts from my iPhone, including uploading photos.  The other week I was at a concert, took a photo of the performer, and uploaded to Facebook on the spot.  It's a fun way to quickly share what you're doing, and being able to make mobile updates helps me make updates that I wouldn't make otherwise.