starting simple, for example, the athas.org dark sun books. athas is the licensed source of 3.x dark sun materials as per WotC, which license has never been rescinded.
birthright, spelljammer, and planescape all have similarly licensed sites and pdf releases.
the mystara site has declined to produce anything new for certain reasons.
the ravenloft site was official until the license was sold and some sourcebooks produced by SSS, but may still be worth listing due to the period of being the official source, having produced product during that period, and it being still available for download.
i believe there was also a dragonlance site, but since WotC released their own product, their specific license is likely no longer valid.
i can provide the other sites upon request.