That's an "it depends", as tax policy varies from state to state. So if your state has an income tax (most but not all do) you'll have some filing and payment(s) there which I forgot to mention. Some states or localities (county or city) may require something as well (perhaps a business license and fee if you're working out of your home, but it's often best not to ask about this since you might run into a zoning problem).

As a general rule states don't tend to levy taxes on computer services, but many of them are desperate. Maryland tried this in 2007 but repealed it before it could take effect (geographically it was complete insanity since the two big population centers are adjacent to other states) and in a Google search I found that the governor of Pennsylvania was proposing this earlier in the year. They did it from 1992-7....

Many clients require a business license to avoid having you classified as a Employee. (The IRS doesnt like you. There are a dozen or so traps that they lay for you, easily avoided if you know about them. See http://ohioline.osu.edu/cd-fact/1179.html) Better still, talk to an accountant. In general a business license isnt a huge hassle. The zoning requirements are usually waived if you have no employees and no visiting customers.

This really isn't true, at least on the IRS/fed level. You don't need anything to be classified as a business. It's merely "how you act" -- are you acting as a business, or are you acting as an employee. There's a list of definitions/guidelines to be aware of, but there is no business license you can get to help with this...