The main program of choice is FL Studio, and Reason is the second most popular. More advanced users use Cubase (in addition or alone). I don't know of many people that use other programs such as Ableton, Sonar, Project 5, or Acid, although they're out there.
Be sure to post any FL questions in the official post in the future to help prevent forum clutter. There are loads of knowledgable people around the forum.