Free HDRIs

1,057 CC0 HDRI maps from Poly Haven and Open HDRI, in one search that understands what you mean: skies, sunsets, studios, interiors and cities. Light a scene with a real place. Use them anywhere, no attribution.

Questions

What is an HDRI?
A 360 degree photo of a real place that keeps the full range of its light, from the shadows to the sun itself. Wrapped around a 3D scene it lights your models the way that place would, with real reflections and soft shadows, and it can be the background too. Blender, Unity, Unreal and three.js all take one as their environment.
Can I use these HDRIs commercially?
Yes. Everything listed is CC0: use it anywhere, change it, no attribution. Each one downloads here at 1K and 2K, as EXR and most as HDR, and links to its source for larger sizes.
HDR or EXR?
Either works almost everywhere. EXR files are a little smaller and keep more precision; HDR is the older format some tools still prefer. For lighting a scene 1K or 2K is plenty; a larger size only matters when the HDRI is also the visible background.