I absolutely love these pages - they give a good detailed comparison of the theatrical, speical edition and dvd version of the original trilogy. A good way to see how much better the dvd transfer is as well.
SW: http://www.dvdactive.com/editorial/articles/star-wars-the-changes-part-one.html
ESB: http://www.dvdactive.com/editorial/articles/star-wars-the-changes-part-two.html
ROTJ: http://www.dvdactive.com/editorial/articles/star-wars-the-changes-part-three.html
Prequels: http://www.dvdactive.com/editorial/articles/star-wars-the-changes-part-four.html
Mod Edit: Unfortunately, the dvdactive.com site is no more - though the original articles linked above can now be found via the Internet Arhive’s WayBack Machine…