There's a list of 5 I really want to add to my repertoire:
French: So I can finally go to grad school for European history and work my way towards a professorship
Arabic: Useful for my more realistic career path, given my major in college
Chinese: As others said, handy business language for the near future
Russian: Because it has always intrigued me
Irish (Gaelige): It is intriguing like Russian, but sounds a lot cooler
But I have no idea which one of the five I would pick.
EDIT: I am already semi-fluent in Spanish and know at least some German and Japanese.