Haven't played the game in something like a year, but I can remember some of the more major branches. There are several spoilers, so if you're worried about that, you should probably stop reading.
Prologue: Confronting Aryan: 1.) Kill Aryan (result: Louisa is in the dungeon instead of Aryan, and depending on the route you take, she's either being interrogated/tortured or making a deal with Shilard)
2.) Keep Aryan alive (result: Aryan is in the dungeon... seem to remember one route out of the dungeon means finding him sitting down somewhere, while another has him being tortured/coerced to sign a document)
Chapter 1: When Roche attacks Iorveth after the two of you confront Letho: 1.) Give Iorveth his sword (result:
lots of violence and fire in Flotsam as non-human rage erupts)
2.) Don't give Iorveth his sword, which means punching him in the throat (result: not only is this hilarious and totally Liam Neeson-esque, but Flotsam turns into a party and almost everyone is happy-slash-drunk)
No matter what you choose, you can still go with either Roche or Iorveth toward the end of the chapter, though I seem to remember actually getting to the characters is a bit different depending on what you chose. For example, if you don't give Iorveth his sword, I'm pretty sure he gets captured, meaning you find him captured... somewhere. The harbor, I think. Sorry, it's been awhile. One thing I definitely remember, however, is that the dialogue will be a bit changed for the combination you choose. For example, punching Iorveth in the throat and then going with him for chapter 2 means that he'll be a little bitter toward you, while giving him his sword and then going with Roche will net you a scene where
Roche erupts at you. A small bit of the dialogue in chapter 2 is changed for this decision, as well. Also, there's a scene late in chapter 1 where you take control of Triss as she talks to someone magically. I'm pretty sure this decision determines whether you talk to Phillipa or Dethmold (which changes the focus of the dialogue a bit), though I'm not 100% on that.
Choosing who to go with at the end of the chapter: 1.) Choose to go with Roche (result: you kill Loredo and
his mom, chapter 2 takes place mostly in Henselt's camp, and a lot of the quests differ from Iorveth's path... you also get a special quest in chapter 3 with Brigida, a werewolf, and some information about what happened to Anais and Boussy)
2.) Choose to go with Iorveth (result: chapter 2 takes place in Vergen and a lot of the quests differ... and there are lots of dwarfs. Most people prefer Vergen and its quests over Henselt's camp. A special quest in chapter 3 opens up where you re-meet both Cynthia and some alchemists from chapter 1. Also, going with Iorveth opens up the below choice at the end of chapter 2)
If you chose to go with Iorveth at the end of the chapter: 1.) Go after Loredo after he sets the building on fire (result: I think you go on and kill him, but some elf women are killed and I seem to remember Iorveth not being happy because he super-secretly wanted you to prioritize them over killing Loredo... you also lose a sex scene in chapter 2 with one of the elf women in the tower, who is obviously too busy being dead to get freaky with you)
2.) Save the women in the tower (result: Loredo stays in control of Flotsam, but Iorveth is happy and you get the opportunity to have that sex scene in chapter 2...
the elf woman also sells ingredients, which can be helpful when you use a lot of potions)
Chapter 2, Iorveth's path: When everyone turns against Prince Stennis: 1.) Let them kill him (result: a fun and probably justified mob murder... and you get the royal blood you need for one of your quests, because it's like... everywhere)
2.) Don't let them kill him (result:
Stennis eventually gets sworn in as king or something in a later cutscene, as memory serves, and you have to get the royal blood you need from Henselt, which
basically means putting a sword to his throat... truly a fun bit of dialogue)
Chapter 2, Roche's path: When confronting Henselt and Roche shows up: 1.) Let Roche kill him (result: Roche is pretty dark from then on, basically becoming a kingslayer like the person he's after, but you also feel warm and fuzzy inside for killing that bastard... later on, Radovid pretty much guesses what happened and you can choose what to say, but it doesn't seem to change anything and the dialogue isn't too interesting)
2.) Keep Roche from killing him (result: I'm not 100% on what happens since I've only done this once, but obviously some of the dialogue changes and Henselt is present at the negotiations in chapter 3)
Chapter 3, Iorveth's path: When faced with the choice to save Triss or help Iorveth: 1.) Help Iorveth (result: Letho saves Triss, but she's not present at the negotiations to speak up, so things become very bad for magic users...
bodies everywhere in the streets... but you have a magic dagger that allows you to break the spell on Saskia after the dragon fight)
2.) Help Triss (result: you
finally get to see Shilard die, and Triss is present at the negotiations to make things better for magic users... at the end there are magical plants and stuff growing around, which is hard to describe... later on in the epilogue, you see a very beat up Iorveth who's pissed at you... I think he got the dagger, but this is at a point where it's too late to use it)
At the end of the quest with Cynthia: 1.) I don't remember the choices exactly, but there are a few different results. The first is that you fight both Cynthia and the male mage with her. The second is that you and Cynthia fight against the male mage together (and she's romance-able if you find her later). The last one... I think you can just let them have what they want and let them go. I'm a bit fuzzy on this part of the game, but that sounds about right. There are some notes that are supposedly dangerous in the Emperor's hands, but whether you keep them or hand them over doesn't seem to have any impact on anything. There's also a magical device that you can use
to see what other people are doing in a short cutscene. There are a few options as far as people to spy on, with a few being more interesting than the others, and I'm pretty sure choosing the option where Cynthia and you fight together means she lets you use it more than once.
Chapter 3, Roche's path: When faced with the choice to save Triss or help Roche kill Dethmold: 1.) Save Triss (result: Triss is around to make things better for mages at the negotiations and Shilard dies, but
Dethmold gets away... I think Roche saves Anais, but I'm not sure about that... but yeah, magical plants and stuff in the epilogue, same as when you save Triss on Iorveth's path)
2.) Go with Roche to kill Dethmold (result: Anais is saved, Dethmold is brutally killed, and you have the option to hand Anais over to either John Natalis, Radovid, or... I think you can tell Roche to run off and take care of her himself... Triss is saved by Letho, but isn't present to speak up for mages so there's lots of violence against mages like when you don't save Triss on Iorveth's path)
Chapter 3, both paths: When Sile asks you to remove the crystal: 1.) You don't (result: she explodes and you feel bad because she was nice and donated money to orphans)
2.) You do (result:
she tells you where your long-lost love is and leaves peacefully before her dragon, which she conveniently didn't call off, attacks you)
When faced with the choice to either kill or spare the dragon: 1.) Kill her (result: you're a soulless douchebag and a little dialogue changes in the epilogue)
2.) Spare her (result: you're a good person, and a little dialogue changes in the epilogue)
3.) If you're on Iorveth's path and acquired the dagger, then you can use it on her (result:
she reverts to Saskia and the two of you talk a bit... and you discover that she has a thing for dwarves)
Epilogue: When confronting Letho for the last time: 1.) Fight him (result: you get into another fight with him, but this one is easier since you've leveled up quite a bit since the first fight.... then you kill him and it's sad because he's like a talking teddy bear)
2.) Let him go (result:
he's surprised, but leaves peacefully)
Anyway, that's all I can think of. There's probably some more, but that should cover most of it.
