3D nature HDRs to light up your scene and provoke various moods. It's all about light in nature.
Skydome rendered with Terragen:
Dusk with a few clouds; those in the higher layer just catching the first/last rays of sun to give interest. Illumination is even with a cool cast.
Lowered horizon, basic ground viewed from a standing height of 1.7m; not obvious if seen from higher.