Here are some interesting Friends facts you may or may not have known about some episodes or the cast and crew.
(Some may be incorrect.)

TOW George Stephanopolous...

* When Phoebe, Monica and Rachel were out on the balcony, Rachel knocks her pillow over the edge. That wasn't really supposed to happen, but they left it in anyway.

TOW Five Steaks and an Eggplant...

* Matt LeBlanc was according to script supposed to grab the phone when Jade called and say "Bob here!". But when jumped for the phone he missed and fell and the editors chose to go with that instead of Joey answering the phone.

TOW Phoebe's Husband...

* A cut line: When Rachel is jealous of Julie, she says with a phony voice "Julie's so smart, Julie's so special". Originally she added "Does she put on a cape and fight crimes at night?"

TOW the Baby on the Bus...

* Giovanni Ribisi (Frank Jr) appeared in this episode as the guy who drops a condom in Phoebe's guitar case.

TO After the Superbowl Pt 2...

* Monkey and Katie (the monkeys who played Marcel) actually played the monkey in "Outbreak".

TOW the Ultimate Fighting Champion...

* You remember the appearance Billy Crystal and Robin Williams did in the beginning of "TOW the Ultimate Fighting Champion"?? That wasn't in the original script. They were by coincidence in the same building-, or the one across the street from where "Friends" is shot and the writers asked if they wanted to make a guest appearance. This was apparently the day of shooting, so it was just quickly thrown in.

TOW Rachel's New Dress...

* Joshua Bergen's dad was played by Matthew Perry's dad.

TOW Ross's Wedding...

* A cut line: It was the scene when Rachel realized she still loves Ross. Originally it was:
Phoebe: "God, that would be like telling Monica, "Hey, you like things clean." Or, you know, hey Joey, you're gay.
Rachel: What!?
Phoebe: Oh, please, she's always got a broom in her hand.

But they just kept it at:
Phoebe: God, that would be like telling Monica, "Hey, you like things clean."

TOW Everyone Finds Out...

* This is what Chandler and Monica says at the start when Phoebe sees them through the window:
Ch: Look, we have to talk. Your shirt is really bugging me.
Mon: Wow, that's so weird 'cause your pants are really bugging me.
Ch: Well, let's make each other happy and get rid of them.

* One of the reasons the audience laughed so much in this episode is when Joey told Phoebe it wasn't the first time he opened a blouse like that is that it took MLB around twelve times to get it open.

* A Chandler and Monica line that was never used:
Ch: Why is it so hard to believe that Phoebe is attracted to me?
Mon: It's not. I believe all the girls are attracted to you because you're part bunny.

TO In Vegas...

* The one and only time they've taken down the Central Perk stage was in "TO in Vegas" when they needed that part of the stage for Caesar's Palace.

* Phoebe says Joey's pin number is 5639. If you look at the letters under the numbers on a telephone, 5639 spells out 'Joey'.

TO After Vegas...

* To commemorate Courteney Cox's marriage to David Arquette, the entire cast is credited with Arquette after their names.

* In the original script for "TO After Vegas" Monica's reply to Chandler's "oh my god, is EVERYBODY getting married?" "I don't know, but we were next!".

* The editor has got laughs on a computer that he can add to the episodes. This was, for example, used when Rachel said "this is not a marriage, this is the world's worst hangover". In that case they used it because the next line was over-laughter. JA continued to speak before the audience quieted. They changed the laughter into a shorter one because they couldn't use one where JA had waited because that would take too long.

* After Chandler tells Joey that the waitress isn't coming 'cause it's a buffet, Joey's reply in the original script wasn't "oh, here's where I win all my money back!", it was "and it's in trouble".

* The writers and producers weren't sure weather people would understand that Phoebe had once gotten married in Vegas and they ended up having to ask the audience.

TOW Ross's Library Book...

* Australian swimmer, Ian Thorpe, was an extra in Central Perk. You can clearly see him sitting at a table near the right of the screen talking to a woman.

TOW the Stripper Cries...

* Courteney Cox was pregnant during the filming of this episode so to prevent her from getting overheated while she was wearing the fat suit, they had a tube hooked up to an air conditioner that they stuck under the costume and down her shirt so that she could cool down between takes.

* All the names that Ross lists at the start of his and Chandler's reunion were actual names of friends and family of David Crane and Marta Kauffman.

Miscellaneous (Show)...

* The episode title format ("The One With/Where/etc...") was created when the producers realized that the episode titles would not be featured in the opening credits and therefore would be unknown to most of the audience. They believed that sitcom audiences generally refer to specific episodes of a show by the most memorable event of the episode (e.g. "Did you see the one where the monkey gets away?") and decided to name their episodes in that format.

* Alternate titles to the show were "Across the Hall", "Once Upon a Time in the West Village", "Six Of One" and "Insomnia Cafe".

* The opening credits pool clips were shot at the Warner Brothers lot at 5 in the morning.

* The frame around Monica's peephole was from the beginning a mirror, but a crewmember smashed it.

* The reason why Ross and Emily's marriage was doomed was because Helen Baxendale was pregnant when she was cast. First she used a corset, then they only showed her from the neck up.

* A break up scene for Ross and Julie was written, but the writers decided to skip it, because everyone's hearts would be with Ross and Julie if they kept it in, instead of with Ross and Rachel.

* Every year five new musical bit is given to the music-people, so that the music will change every now and then.

* Throughout the series, 27 episode titles reference Rachel, 23 reference Ross, 15 reference Joey, 10 reference Chandler, 10 reference Phoebe and 8 reference Monica.

* The character of Ursula (Phoebe's twin sister) first appeared in the sitcom "Mad About You" as a waitress. Kudrow played Ursula on both "Mad About You" and "Friends". The shows creators did not originally intend for Phoebe to have a twin, but they added the character to explain why Kudrow was appearing on two different shows on the same network, at one time as a part of the same Thursday night line-up.

* Although it seems that most of the action of the series took place in Greenwich Village in Manhattan, New York City, the series was filmed in California. The exterior shot of the Friends apartment building is located at Grove and Bedford streets in the Village. No scenes featuring cast members were filmed in New York; even the show's opening is shot around a fountain in California.

Miscellaneous (Actors)...

* Jennifer Aniston says her favourite food is mayonnaise on white-bread sandwiches, and that she loves to make baklava.

* The name Matthew Perry uses when checking into a hotel is 'Abe Flamon'.

* The name Jennifer Aniston and Brad Pitt use when checking into a hotel is 'Mr. and Mrs. Ross Vegas'.

* The reason Monica cut her hair so short in season four was because Courteney Cox Arquette had those streaks in her hair from "Scream 2" and they didn't exactly match Monica's character.

* Courteney Cox actually auditioned for the part as Rachel, but they asked her to play Monica instead, since Monica's the "main" character and they wanted a well-known actress for that part.

* Matt LeBlanc's role in "Lost in Space" was from the beginning offered to Matthew Perry. He declined, however, but tipped the casting people about MLB. And that's how Matt got the part.

* David Schwimmer was offered Will Smith's part in "Men in Black" but turned it down.

* Chandler got his first on-screen kiss by Jill Goodacre Connick. She is married to Harry Connick, who starred in ID4 as Will Smith's friend who dies in the first air battle with the aliens. That part was first offered to Matthew Perry.

* Joey hurting his arm and having it in a cast was not part of the original storyboard, it was just thrown in. Matt LeBlanc dislocated his shoulder on the set and that's how Joey ended up falling of the bed and hurting his arm.

* Carol's obstetrician in TOW the Birth was played by David Schwimmer's best friend Jonathan Silverman.

* Terry, Central Perk's manager, is played by the dad in "Alf".

* Alan, the guy Monica dated and the others loved, is played by the priest from "Dr. Quinn".

* A funny note is that David Schwimmer played a student guidance counselor in "Apt. Pupil" and he mentioned Brad Renfro's character's parents being named Monica and Richard.

* Reese Witherspoon was signed up for around six episodes, but apparently Jennifer Aniston disliked her and asked the producers to fire her.

* Jennifer Aniston had a painting exhibited at New York's Metropolitan Museum of Art when she was just 11.

* Matt LeBlanc once won an award for a tomato sauce (Ketchup) commercial.

* The first female member of the cast that Matthew Perry ever kissed (as far as we know *wink, wink* lol) was Jennifer Aniston, and not Courteney Cox Arquette as most seem to think.

* David Schwimmer directed several episodes.

Thanks to: Annie... for sending me interesting facts.
Some interesting facts taken from Wikipedia.