Here's a good post from Small Biz Trends outlining 10 Useful Apps for the Small Biz iPhone user.

Have a look at the details of each here: 10 Useful Apps 

Keep track of time you work on projects withMinibooks. 

Plan your taxes ahead of time with Contracter, Freelancer, Small Business Tax Calculator. 

Manage your taxes with Opcenter.view- Quickbooks Desktop Data on the GO. 

Manage your business contacts with the LinkedIn app.

Organize tasks with EasyTask Manager.

Keep track of your customers via Salesforce Mobile.

Keep track of payroll through SurePayroll.

Create visualizations of your business data with Roambi-Visualizer.

Get hundreds of top business book synopses with Top Business Summaries.