The first big change is the final confrontation with Saruman (after an extended meeting between Merry & Pippin and the others). Saruman and Wormtounge are on top of Orthanc, Gandalf tries to get information out of Saruman, Theéoden tries to get Wormtounge to come back to the good side, Saruman tells them that they're all going to die. Wormtounge stabs Saruman and gets shot for his trouble, they both die.
Back at Edoras, the celebration is extended with Gimli and Legolas having a drinking game and some character moments (including Éowyn hiting on Aragorn some more).
At Minas Tirith, there's more of Gandalf and Pipping talking about the decline of Gondor, another scene of Denethor berating Faramir for not being Boromir, and a scene between Faramir and Pippin before he swears allegiance.
On the road to Mordor, Frodo and Sam find a ruind and grafittied ancient Gondorian statue, and Sam gives Gollum a stern warning on the Stairs of Cirith Ungol.
The Paths of the Dead has Gimli blowing off some ghosts, then Aragorn et al escaping through a river of skulls and Aragorn wibbling on Legolas when they get out before the Dead actually take their side. We then get to see them kill Peter Jackson and capture the corsairs' ships. Meanwhile Merry is hitting on Éowyn.
Back at Minas Titith, theres a conforntation between Gandalf and the Witch King - The Witch King destroys Gandalf's staff and is only drawn off by the arrival of the Rohirrim. After the Battle of the Pelennor fields, there's extended
Over in Mordor, we get to see Sam and Frodo hooking up with a band of Orcs, and the appearance of the Mouth of Sauron ath the Black Gate, complete with Frodo's mithril vest. After this point there's only one scene that's marked as extended, and certainly the ending is not drawn out any further (and, in fact, given the considerably longer overall run time, doesn't seem so long).
Now I'm off to start watching the commentaries...
