1. Original Theatrical 70mm 6-track mix
2. Original Theatrical 35mm Dolby Stereo
3. Original Theatrical mono mix
4. 1985 digitally remastered mix
5. 1993 THX-certified digitally remastered mix
The original 70mm and 35mm Dolby mixes are very similar - the 70mm obviously having a "weightier" presence, but there are a few spots where the effects mix differs noticeably.
The mono mix was redone from scratch, and features the addition of stormtrooper and C-3PO lines, additional sound effects, and different ADR (e.g. Beru's dialogue and Luke's "Blast it Wedge"). See The Starkiller's Guide.
The 1985 digitally remastered mix was based on the Dolby Stereo mix. Threepio's line from the mono mix was added, and it was mixed with a wider stereo image (possibly to address the overly conservative approach used in early Dolby Stereo mixes).
The 1993 mix, as found on the '06 DVDs, was supposedly a combination of the best elements of the original 3 mixes, but mostly based on a downmix of the 70mm version. Some of the effects from the mono mix were added in, but they forgot the additional stormtrooper and C-3PO lines. Some additional new material was also added - see this thread for a list of differences between the 70mm and the '93 mixes.