For the purpose of information and research, I threw together a list of all the versions of the Star Wars films that have been released, and what is currently the best known DVD source for each film.
Note: I am only including visual differences here - a list including all the different audio versions would be substantially longer. Also, I have not taken into account all the various different non-English releases.
Star Wars film versions:
Star Wars (original theatrical version)
Best source: widescreen bootleg telecine transfer
Best simulated source: official 2006 [G]OUT DVD (original crawl spliced into 1981 version)
Star Wars (1981 version)
Differences: "A New Hope" subtitle added to crawl
Best source: various laserdisc transfers
Star Wars Special Edition (1997 version)
Differences: various added scenes and digital alterations
Best source: TB digital broadcast transfer
Star Wars Special Edition (2004 version)
Differences: further digital alterations added
Best source: official 2004 DVD
Best HD source: WookieGroomer's 1080p WMV files
Star Wars Special Edition (2011 version)
Differences: even further digital alterations added
Best source: official 2011 Blu-ray
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The Empire Strikes Back (original theatrical version)
Best source: official 2006 bonus DVD
The Empire Strikes Back (70mm version)
Differences: various minor scene and editing differences
Best source: [none known]
The Empire Strikes Back Special Edition (1997 version)
Differences: various added scenes and digital alterations
Best source: TB digital broadcast transfer
The Empire Strikes Back Special Edition (2004 version)
Differences: further digital alterations added
Best source: official 2004 DVD
Best HD source: WookieGroomer's 1080p WMV files
The Empire Strikes Back Special Edition (2011 version)
Differences: even further digital alterations added
Best source: official 2011 BD
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Return Of The Jedi (original theatrical version)
Best source: official 2006 bonus DVD
Return Of The Jedi Special Edition (1997 version)
Differences: various added scenes and digital alterations
Best source: TB digital broadcast transfer
Return Of The Jedi Special Edition (2004 version)
Differences: further digital alterations added
Best source: official 2004 DVD
Best HD source: WookieGroomer's 1080p WMV files
Return Of The Jedi Special Edition (2011 version)
Differences: even further digital alterations added
Best source: official 2011 BD
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The Phantom Menace (press screening version)
Differences: Darth Maul does not split in half when falling down the shaft
Best source: [none known]
The Phantom Menace (original theatrical version)
Best source: TB digital broadcast transfer
The Phantom Menace (extended DVD version)
Differences: various extended scenes
Best source: official DVD
Best HD source: WookieGroomer's 1080p WMV files
The Phantom Menace (extended BD version)
Differences: various extended scenes plus further digital alterations
Best source: official BD
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Attack Of The Clones (original theatrical film version)
Best source: bootleg camcorder transfer
Attack Of The Clones (digital theatrical/DVD version)
Differences: various digital alterations, Anakin takes Padme's hand at wedding
Best source: official DVD
Attack Of The Clones (IMAX edit version)
Differences: various scenes edited down to fit IMAX running time
Best source: [none known, VCD bootleg rumored]
Attack Of The Clones (2011 version)
Differences: order of some scenes changed
Best source: official BD
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Revenge Of The Sith (original theatrical film/theatrical digital version)
Best source: official BD
Revenge Of The Sith (DVD version)
Differences: diagonal wipe to Anakin crawling up the volcanic slope is now a straight cut
Best source: official DVD
Best HD source: WookieGroomer's 1080p WMV files
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The Star Wars Holiday Special
Best source: KCCI broadcast recording
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The Ewok Adventure
Best source: various laserdisc transfers
Caravan Of Courage (aka The Ewok Adventure)
Differences: "The Ewok Adventure" title card changed to read "Caravan Of Courage"
Best source: official DVD
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The Battle For Endor (aka Ewoks: The Battle For Endor)
Best source: official DVD
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For a detailed rundown of the differences between movie versions (with screenshots), see Chris Gould's articles here.
--SKot