If you need more than what iOS’ built-in Contacts.app allows you to do, Greg Pierce’s new app piece of wizardry is for you:
Interact empowers you and your contacts to get more done in less time, with features like group management, one-tap contact creation, and more you won’t find in the built-in iOS Contacts app.
I was able to beta test this app, and it really is amazing what you can do with your contact data right from an iPhone with this app. Go check it out.