"Star Wars", now Episode IV: A New Hope (I hate even the name revisionism).
I think there were two sets of changes, the first the accompanied the rerelease, including the title change and change in the opening crawl text, and some CGI animated scenes particularly around Mos Eisley spaceport, miscellaneous animated figures, womp rats, storm troopers riding big lizards etc.
I personally didn't feel these scenes added anything at all to the film and in fact clashed a bit with the lower tech look of the old movie.
The thing that PO'ed me the most aside from the change in the opening crawl text was the cantina scene. If you remember the way it went down in the original, Han Solo is leaving the cantina having just made a deal with Obi Wan and Luke to carry the droids. Greedo the bounty hunter stops him and forces him to sit back down, at gunpoint. They discuss Han's debt to Jabba while Han surreptitiously readies his blaster under the table. Greedo discusses confiscating Han's ship, Han says "over my dead body", then Greedo says something to the effect of "that's the idea" and Han blasts him from below the table, leaving a smoking Greedo slumped over the table. Then Han flips the barkeep a coin ("sorry about the mess") while walking, slowly, out of the bar while the band continues to play.
In the DVD release, the scene is altered to make sure to show Greedo "shooting first" so that Han would not be blasting a guy who wasn't already shooting. In an interview, George Lucas said that basically he was "correcting" the original, because Han shooting Greedo in this way was not consistent with his being a good and admirable character, someone to emulate.
Well why the hell did he write and shoot the scene this way in the first place?! Obviously because it was a cool scene that worked, and was entirely consistent with the pirate character of Solo dealing with treacherous scum on a daily basis. I remember this as a great scene in a movie packed with memorable scenes. Really got a rise out of me and the rest of the audience when we originally saw it. I only wish that people like Lucas remembered what they were thinking and feeling when they originally wrote this stuff...
(I haven't seen it but I understand that the DVD release of "E.T." changed a scene where the space suited NASA dudes closed in on E.T. at Elliott's house, and were holding guns, the guns changed to walkie talkies because the guns were too scary for little kids or something. That's exactly the point! The scene is scary! When the gov't dudes come to imprison and experiment on your space dudes, they come with guns AND walkie talkies! The dudes with walkie talkies coordinate the gun guys! Anyway, I've heard that the original E.T. is available as well as this version, but not in the case of "Star Wars".)