This mod is called Vilja in Skyrim, some people might remember Vilja from Oblivion, well now she has arrived in Skyrim :)
Basicly Vilja is a companion mod, but ther is so much more to her than just a companion that follows you around and blocks the doorway you wanna go through (like Lydia does all the time).
This companion feels much more "alive", she actually interracts with the world, she will make remarks about things she sees, give comments about quests you are on or give her opinion on quests you have done.
In fact she talks a lot, and by a lot, i mean A LOT :)
Sometimes she is annoying, sometimes you will laugh, she talks a lot to you, to herself or to other companions (like Lydia, Aela and many other..), most companions she will actually hold conversations with, i have not tested with all but she will actually hold whole conversations with Lydia and Aela.
One of the coolest features (i think) is that you can actually ask her what she wants to do next, or you just ask her to take you somewhere (like a city, nearest tavern or let her decide on some random location) and she will lead the way and bring you there.
Now that feels really awesome, instead of you taking the lead and your follower just following you, you are now following your companion to where she wants to go.
She will every now and then tell you that "it isn't fair that you always get to decide where to go next".
She is fully voiced with over 4000 lines of dialog with a cute swedish accent :)
There is way more awesome stuff with this mod and if you want to know more you can read it on the mods page on the Nexus.
All in all this makes her feel very much "alive" and not like some stupid mute robot following you around.
I admit that at times she can be quite annoying, as she (by default) doesn't shut up, i like her that way, but you can, in her menus, adjust her setting so she does talk less.
Here's a little video that will give a much better review of Vilja than i did in this post :)
For me this companion is so much fun, that i have left all others at home, whenever i just take Lydia with me it feels like i am missing something :)