Contact (1997), a movie starring Jodie Foster, is based on CarlSagan's bestselling novel (1985) of the same name (not BillionsAndBillions).
The story is about the first contact the human race has with extraterrestrials (via radio signals) and touches on how science, religion, and politics respond to the news of the signal and what it carries.
The movie was directed by Robert Zemeckis, who also directed Forrest Gump.
CarlSagan and his wife Ann Druyan were co-producers of the movie, but Carl died before the movie was completed (hence the film's dedication "For Carl").
Due to the time constraints the movie version has a fair amount of stuff left out that was in the book (the movie version has only one traveler instead of six, and we never witness the perfect circle found within the number pi.) This really doesn't detract from the film, as what's left is used to convey the story very well. Even with the slimmed down timeline and the smaller cast of characters, the movie still clocks in at 153 minutes.
Stock footage of President Bill Clinton (speaking about NASA exploration of Mars) was integrated into the film to great effect.
In 1996 (a week after CarlSagan died) Francis Ford Coppola sued the estate of CarlSagan claiming that the novel and film are based off his idea from 1975 for a TV show called First Contact. Carl was contracted to develop the script for the show but instead developed a book based on the idea, which led to the film. A Los Angeles judge tossed out the lawsuit ruling that the statute of limitations had run out.
The movie has an official web site at: http://contact-themovie.warnerbros.com/main.html
There's also a pretty comprehensive review of it at http://www.coseti.org/klaescnt.htm
When the movie was originally released on DVD, it sold out quite easily, likely due to all the extras packed in.