Throughout the movie, Popeye asks suspects and criminals during informal interrogations, "You been pickin' your feet in Poughkeepsie?" What's up with that? The daughter of Ed Keyes, Robin Moore's author-collaborator on The French Connection book, revealed that it had always been something her father had said. "My father was always asking us if we ever picked our feet in Poughkeepsie," she said in an interview. Where he got it, though, we might never know.
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Chateau d'if, the island fortress where Henri Devereaux agrees to be in business with Charnier and Nicoli, is also the setting for The Count of Monte Cristo. Source)

Simonson is the real Popeye, and Klein is the real Cloudy. The two detectives, Eddie Egan and Sonny Grosso, whose work on the French Connection case was documented in the book, got to appear in the movie… and both went on to careers in film and TV. (Source)

Not only did IRL Cloudy Buddy Grosso play Klein in The French Connection, he also had a minor role in The Godfather, meaning that this dual-career dude managed to appear in two Academy Award-winning Best Pictures in his 10-credit run as an actor. Now he sticks to behind-the-camera work as an executive producer. (Source)

The Europe-based clothing label French Connection was started in 1972, a year after the release of the movie, after a British businessman met a French designer. He doesn't have any particular comment about its, um, connection to the movie. (Source)