From Wikipedia

Official material for 3rd Edition
While the campaign is no longer supported with published rulebooks, rules for the 3.5 edition (d20) have appeared in several places. Both Athas.org and Paizo's renditions of Dark Sun are official versions approved and sanctioned by Wizards of the Coast, even if they are mutually exclusive, and provide two different renditions of the setting. Some have argued that the products are not mutually exclusive, but simply refer to two different time periods in the world, hundreds of years apart. Two of the authors of the Paizo materials, Chris Flipse and Jon Sederquist, are on the Athas.org "overcouncil," and are responsible for the much of the development of the Athas.org rules.

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Note that very few Athas.org files are fully not playtested, but release candidates. If you find a glaring mistake or something you think should look different, head to http://athas.org/ to check up on the file. I've made a list of file versions in the torrent for comparison following:

Rules material

    * Dark Sun 3 (Revision 6, Released 2005 July 4)
    * Terrors of Athas (version 0.9, Released 2004 February 27)
    * Terrors of the Dead Lands (version 1.2, Released 2003 May 27)
    * Prestige Class Appendix Volume I (Revision 2, 2005 December 05)
    * Prestige Class Appendix Volume II (beta review 2, 2007 November 30)
    * Advanced Beings (Dragons) (Revision 2, Released 2005 December 22)
    * Advanced Beings (Avangions)
    * Champion of Rajaat (Revision 1, Released 2005 December 22)
    * Athasian Emporium (Preview 1, Released 2007 December 17)

Setting Material

    * City State of Draj (Version 1.0, Released 2003 August 21)
    * Trade Lords (Version 1.0, Released 2007 January 29)
    * Villages of the Wastes (version 1.1, Released 2000 September 9)
    * Wisdom of the Drylanders (Release 2, Released 2006 September 30) 

Adventures

    * Tyrian Conspiracy (Version 1.2, 2005 November 27)
    * Whispers of the Storm (Version 2.0 released 2006 March 03)
    * Dregoth Ascending (Updated 2006 March 03) 

There's always a lot of talk at the Dark Sun WotC forum: http://forums.gleemax.com/forumdisplay.php?f=290

Athas.org also links to a number of Dark Sun flavor sites.