I know we used to have a topic about this, but I cant find it! Soooo I thought it'd be useful to keep all we find out about the Extended Edition in one place!
Ive read several sites and articles, and as usual TORN offers the most complete synopsis

Some of which, of course, is supposition..... (Courtesy of Datacomm and transalated by those at TORN

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A beautiful scene, before the two hobbits, guided by Gollum, walk on… The hobbits catch sight of a huge monument and statues. Up to this scene the sky wasn't edit digitally… from now on it's getting darker and darker … a cursed environment. In this scene you see the title of the movie "The Return of the King"
- Saruman's disempowerment:
Saruman's last dialog with Gandalf, Theoden and the others will only be seen on the SEE. Saruman is going to appear on the balcony, trying to bewitch the fellowship one last time by talking to them. But Gandalf breaks Saruman's wand. After that Wormtongue and Saruman appear on the roof of the Orthanc. Saruman treats his servant like a dog and threatens him with a knife. But Wormtongue fights back, grabs the knife and throws Saruman from the Orthanc. His own weapons pierce Saruman and Legolas kills Wormtongue with one of his arrows.
- Edoras: Merry's vow of fidelity:
During the whole movie Merry serves the King, Theoden. But why? Like it was shown in the first teaser of the trilogy: the king dubs Merry knight of Rohan. Merry offers his services and Theoden accepts and hands him a sword.
Dialogue:
"I offer you my service, King Theoden" Merry is given a knighthood.
- Edoras: Dialogue between Eowyn and Eomer concerning Merry:
Like we have seen in the cinema version, Eomer can't believe that Merry is able to take part in a battle. There will be a typical brother-sister scene on the SEE, concerning this topic. The existence of such a scene is confirmed by a "making off video", because Peter Jackson mentions a "Brother and Sister" moment.
- Edoras: A "drinking game" during the celebration:
After the success in Isengard, Aragorn, Merry, Gimli, Theoden and the others go to Edoras. There will be some new elements in the scene were both hobbits dance on the table while Gandalf talks to Aragorn about Frodo... the SEE will show another funny scene (which isn't in the book) . Legolas and Gimli play a drinking game...
Dialogue:
Legolas: "So, it's a drinking game?"
Laughing and screaming of the others
Legolas: "What exactly is the point of it?"
Gimli: "Last one standing wins."
- Edoras: Extended discussion:
In Edoras Aragorn and Gandalf talk about their proceeding. There will be more dialogues than in the cinema version.
Dialogue:
Gandalf: "If the ring is destroyed, Sauron will fall and his fall will be so low that none will see his arising ever again… but if Sauron regains the ring, his victory will be swift and complete. Things are now in motion that cannot be undone. Sauron has looked into the face of young Peregrin Took and mistaken him for the Ringbearer".
Aragorn: "He has gone unchallenged long enough."
Merry says good bye to Pippin, who seems to be very sad. Merry bolsters Pippin and tells him that they will see each other in the shire. As far as I know, a new scene replaced this one. We won't see this dialogue on the SEE and we won't hear the sentence "We shall see the Shire again."
- Edoras: Extended Farewell:
After Gandalf and Pippin left, Aragorn tells Merry, that Pippin will only be save at Minas Tirith, because Saruman thinks he is the ring bearer. That's why Gandalf has to bring him away. In the cinema version, Aragorn doesn't say anything. Before Pippin leaves Edoras, Aragorn gives him back his elven leaf brooch.
The scenes between Arwen and Elrond in Rivendell will be longer. As seen on the trailer, Elrond tells his daughter that he is not able to protect her anymore. There is another scene where Arwen falls into her fathers’ arms. Is there a connection between this and the paths of dead she discovers in her book? Arwen will also say some more words in the language of the elves.
Dialogue:
Elrond to Arwen on the bridge: "You have given away your life’s grace. I cannot protect you anymore"
These two dialogues are not in the cinema version: Arwen says: "Give him the sword of the King"
Elrond: "The enemy will never let Aragorn come to the throne of Gondor."
- Extended Paths of the dead:
There is supposed to be a corridor in the cave, full of skulls and bones on the floor and left and right in the walls. At one point Aragorn, Gimli and Legolas will be threatened by an avalanche of skulls. After leaving the cage there has to happen something else, because Aragorn’s sword looks bloody. Legolas is also supposed to have some more sentences.
- Legolas will kill Peter Jackson, who supposed to have a cameo appearance as a corsair captain.
In the cinema version you only see a little bit of the paths of the dead, no skulls, no altars, no bloody sword, no corsair ships. I am sure they will be shown here, because Gandalf talks about them twice.
After the paths of the dead Gimli, Legolas and Aragorn board the ships and conquer the corsairs. But which scenes do we see on these two pictures? Which scenes will be shown on the SEE? Do we see how they board the ships? Yes, but without the ghosts, because they will appear later, as a surprise, on in the Pelennor Fields.
- Minas Tirith: Longer dialogue between Faramir and Denethor:
Faramir is supposed to tell his father that he did let Frodo and Sam escape (with the ring) on purpose, breaking his father's rules.
While Pippin talks to Denethor, Faramir stands at his side. Shortly after that Denethir announces that those who break the rules have to be punished. A hint that Faramir really told him about the hobbits? A script section asserts that there will be a flashback, where Faramir tells his father about Boromirs death and hands him the broken horn. In another scene Pippin is going to get an armor from Faramir.