Can someone explain the last cut scene for me. I'm assuming its Cortana talking to Chief, and the thing you see in the background is the half of the spaceship from Halo 3...but then again I don't know.... :/
 

Its Cortana talking to 6. She's making sure you know that what you did in this game wasn't pointless. This shot is also 30 something years later and Reach is being rebuilt. I don't if that helped very much but that's how i took it.

It's Cortana? I thought it was Halsey, why the hell did I think that?

I thought it Halsey too.  
 
I think that ship was just supposed to be some wreckage. Throughout the campaign, there were plenty sights of destroyed or heavily damaged ships just littering the skybox or the ground. I don't think there was anything more to that. Halsey was simply saying that our effort wasn't in vain, because our actions enabled the end of the war allowing humanity to rebuild.

" @MaFoLu:  Yeah I thought it was Halsey too at first but isn't she supposed to be trapped in a Dyson Sphere that was at the core of Onyx? "

Yeah, Halsey. Also the ship is there to show that the planet is being reconolized.

Isn't Cortana modeled after Halsey, with the same voice an' all? I can't quite remember.

I'm pretty sure the subtitles said it was Halsey talking during that scene

" Isn't Cortana modeled after Halsey, with the same voice an' all? I can't quite remember. "

It's Halsey. Cortana only has 7 years of operational life before she becomes rampant and thus wouldn't be coherent.

Yes, it's Halsey, and yes, that ship in the background is just to show how Reach has rebuilt after the war.
 
I do hope we find out some day how Doctor Halsey escaped Onyx. Maybe that's where the Master Chief was heading at the end of Halo 3? Would be a great story for 343's first Halo game.