Just a couple.
The Official Site’s old “What Has Changed” articles…
1997 SE changes - http://web.archive.org/web/20110623222820/starwars.com/episode-iv/bts/article/f19970115/index.html
2004 SE changes - http://web.archive.org/web/20070316071040/http://www.starwars.com/episode-iv/release/video/f20060825/index.html_
1997 SE changes - http://web.archive.org/web/20100328044359/http://www.starwars.com/episode-v/bts/article/f19970115/index.html
2004 SE changes - http://web.archive.org/web/20080221004017/www.starwars.com/episode-v/release/video/f20060901/index.html_
1997 SE changes - http://web.archive.org/web/20100329231926/http://www.starwars.com/episode-vi/bts/article/f19970115/index.html
2004 SE changes - http://web.archive.org/web/20071017002726/http://starwars.com/episode-vi/release/video/f20060908/index.html_
Over at theforce.net, this one’s old as well, and has small screenshots…
Paul Ens’ Star Wars Special Edition Annotations or http://web.archive.org/web/20041204003859/http://www.theforce.net/swse/
Brand new, has large shots of all versions, very nice…
Sam Davatchi’s page or http://web.archive.org/web/20041208053607/http://perso.club-internet.fr/willow/Index.html