As others have said, it really depends on where you live and what kind of career you're in/planning to go into.
For an American in general, the most useful secondary language by far is Spanish thanks to the massive latino population in the US and the fact that pretty much the rest of the Western hemisphere speaks Spanish (with several exceptions in regards to Portugese, but as it turns out if you know Spanish you can also understand Portugese pretty decently). If you plan on entering a job that has you dealing with a lot of people on a regular basis (retail, law enforcement, MD, etc) then it would probably be in your best interests to learn Spanish. If you live in Canada though then you'd probably be better off learning French thanks to Quebec.
If you plan on going into business then the most useful language depends on which foreign countries your employer interacts with the most (for example, my dad works for the American branch of a French owned company, so for him French would definitely be useful to know since he finds himself having to fly over there several times a year). In general though the most useful business languages after English are Japanese and Mandarian Chinese (note that despite China's growing importance in economics, Japan is still the world's 2nd largest economy and still considered the gateway to the rest of the Asian market).
If you plan on going into some sort of government work, especially military, intelligence, or state department, then the most useful languages to know are Arabic, Persian, Chinese, and Russian. Also note that the US government does hand out bonus salaries for each non-English language you're fluent in, and that the amount of that bonus is dependent on how difficult the state department considers that language to learn (thus Middle Eastern and Asian languages get you the biggest pay bonus whereas Romantic languages like French and Spanish get you the lowest).
If you want to go into engineering then the best languages are probably German and Japanese. Hindi, Korean, and Chinese would also be good choices. The same goes for science in general.
Any kind of programming or other high tech job, definitely Japanese.
Really, the best language to learn is whatever strikes you as being the most interesting and having the most interesting culture. Spanish may be the foreign language most likely to be encountered by an American, but at the same time if you don't find Spanish or Latin American culture and/or the language interesting then you're not going to be very motivated to learn it. I'd say that the only time to learn a language for utility's sake is when you know you're definitely going to need it.
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