Who was Anakin's hologram at the end of Star Wars episode 6?

That is only in the remastered version. It wasn't in the original release.

here ya go

In the 2004 DVD release of Return of the Jedi, the ghostly image of Sebastian Shaw as Anakin Skywalker is replaced with Hayden Christensen.

They edited it after episode 3 came out and replaced the older man with Anakin from episode 3. You have an older version of the movie before it was edited. The whole reason behind why Lucas changed it is that Luke turned Vader back around to from the dark side right before he died. So when he transformed into the force he turned back into Anakin, who he was before he turned to the dark side and became a sith.

It wasn't a hologram, it was the force version. And yes, they replaced the original actor with Hayden Christiansen for the remastered versions. He would have been a little kid when the original was filmed(if he was even born yet)

Ok first off it was a "hologram" or a "force version" as is said in one of the other answers although I suspect that person meant to say "force vision" but I don't know. In fact it isn't either of those "terms" ... those are what is called "Force Ghosts". Remember how both Obi-Wan & Yoda's bodies just disappeared when they died? That's where their Force Ghosts come from. In ep V the Force Ghost of Obi-Wan tells Luke to go to Dagobah. When Luke burns his father as his funeral all he burns is the electronic parts of Vader's machine ... the flesh & blood body of his father had disappeared when he died even though it was not shown in the film.

Next, you have to remember, something that a lot of so called "die hard" fans, that there are actually 3 different versions of the OT (ep IV - VI). There were the originals in 77, 80, & 83. The SE versions that came out in theaters in 1997. This version had some scenes added, some changed, along with jacked up picture and sound. The 3rd version is the DVD version that came out in 2004.

This scene is in ALL 3 versions of the ep VI ... it wasn't one of those "added scenes" it was one that had always been there. In the original and the SE versions the Force Ghost of Anakin is played by the same guy who was the "unmasked Vader" ... Sebastine Shaw. In the SE version of ep VI Lucas had a "DDDUUUHHHHHHH" moment ... in the scene where Vader is unmasked they added bushy eyebrows to Shaw. Why is that a "DUH" moment ... because a person who had as much burn damage to his head and face as Anakin did ... well bushy eyebrows just would never have happened ... lol ... the mind of Lucas is a truly "strange" place ... lol ...

It wasn't until the DVD versions came out in 2004 that the Force Ghost for Anakin in this scene was changed from Shaw to Hayden Christiansen. Of all the changes Lucas made to the OT this is usually # 2 on the "WTF are you doing George" list ... # 1 is the "Who shot first ..." thing between Han & Greedo in ep IV ... don't even get me going on that cluster fu*k stupidity.

When asked why he made the change of adding Hayden in that scene Lucas said he did it so fans could tie the two trilogies together with a familiar face. If he had stopped there most of us old school fans would have grumbled about it under our breaths but we would have probably let George get a way with that but in true Lucas fashion he didn't know when to shut up. His second reason was that a Jedi's Force Ghost was the image that they had of themselves when they died and that Anakin Skywalker had died when he became Darth Vader. Lucas screwed the pooch big time on that one. If he had check his own databanks at SW.com he would have know that in the bio for Anakin Skywalker it says that Anakin died in the arms of his son Luke on board Death Star II.

If you've never seen the original versions of the OT see if you can pick up a copy of them ... they came out on DVD in 2006 but are a little hard to find at the moment at mass retail stores like Wal-Mart, K-Mart etc ... but they can be found pretty easy at Amazon.com. If you do get copies of them I think it will be a wild thing to be able to watch the original version of each of the movies followed by the DVD version ... the differences will blow you mind.

May The Force Be With You ...

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Do you mean this scene? http://en.wikipedia.org/wiki/File:Comparison_of_di...

There were a whole lot of little changes made to the original trilogy. The below link is to the Wiki page listing and talking about them, from which I got the above link.