The list of the top-grossing American films of all time is filled with movies that had (or were) sequels... everything from Star Wars to Home Alone to Jurassic Park. In fact, so many of them appear on the list that we think you'll have a difficult time naming the top-ten U.S. films of all time that were NOT part of a series.
Just to be clear: (1) these movies have neither been nor spawned any theatrical-release sequels to-date; and (2) we're using the films' original box office takes, not a conversion to 2008 dollars. (So, yes, they're all from the last quarter-century or so, hint hint.)
You've got five minutes after you enter your first answer. Good luck!