Terragen’s easy sky generator is now available in Early Access, for Windows desktop PCs. Make “photorealistic CG skies” with “a simplified UI and workflow” using sliders. Also 360-degree HDRIs.
In Early Access it’s apparently not yet a standalone software, but will be. Currently it exports the sky into the main Terragen desktop software, and thus is only available to those who have a Terragen licence or subscription. If Terragen Sky will be free or low-cost and/or public, when it reaches its final release, is as yet unknown.