Technology

The Internet

This research automatically doubles the Tourism output for all of your cities. At this point in the game, Cultural Victory becomes very threatening and hard to counter. If you've been waiting to conquer your rivals, it may be too late unless you hurry to finish things.

By the same note, those working toward a Science or Diplomatic Victory should be lining up their final moves as well. Otherwise, a strong Culture player who finishes researching The Internet is going to beat them to the punch as well.