Accounting and book-keeping, once money starts to flow.
Payroll.
Support (you'll want it taken care of, but you shouldn't succumb to the impulse--at least not entirely, because you need to support customers daily in order to understand them).
Legal crap. Taxes, employment agreements, etc.
I want to say billing and invoicing and payment handling, but I think it's probably more efficient to bite the bullet and integrate these tightly into your own system--so that you always know exactly what's happening and how every change in your product or marketing effects your bottom line.