Check out the Mac App store. There are currently 5 invoice programs on there you can take a look at. You need to be running at least OS X 10.6.6 to access the App store.
There are quite a few software and Apps of invoice programs such as Billable from Clickable Bliss, Express Invoice from NCH Software and Involer from The Dream Apps. But my choice is Zoho Invoice. Their software for invoices can be used on Mac, Android and Iphone. It is a user friendly and has mobile access.
I like Billable but I have no experience with Zoho Invoice. The main reason I like Billable is that it does BOTH time tracking AND nice-looking invoices in one program.
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