I use Freshbooks on the web all the time to send out invoices and request payment for various writing and freelance jobs I have. It allows me to create invoices, send them, and have people respond to me as if I have a real business with real invoices. It’s pretty slick. Other features include time tracking, project management, estimates, and expense tracking.
Now, there’s an iOS app that lets you do the same things, right from your iPhone.