Regulations and US Law provide extensive protection against discrimination in employment. Many companies and organizations you will encounter state explicitly that they follow Equal Opportunity Employer guidelines. Basically, this means that they do not discriminate on the basis of age, gender, marital status, religion, or sexual orientation, to name a few.
The Persons with Disabilities Acts also specifically protects people with physical or mental disabilities or handicaps in addition to providing special accommodations for them (e.g. wheelchair access). Because of these laws, organizations are increasingly careful about the way in which they ask questions in interviewing; no one wants to be the subject of a lawsuit claiming discriminatory hiring practices.
While no specific federal, state, or local entity specifically provides a list of illegal interview questions, there is sufficient precedent (legal history) in court rulings, legislative decisions, regulations, and constitutional laws to govern certain categories of questions. Some of these questions may be perfectly acceptable outside of the US and so, may appear to be benign.
As an interviewee, you should be aware of the types of questions that may be problematic in the US as well as how to handle them in an interview. Bear in mind that both law and precedents continue to change. Therefore, the list of potentially illegal questions (or their legal counterparts) presented here is in no way exhaustive.