V5.1

After timing many test imports I have changed the compression settings on the sky textures. They now import at 2048 resolution and normal quality. This will result in much faster import times for the asset as a whole. But please remember to alter them if you want to maximise quality and reduce artifacts. Do what is most suited to your project's needs. See the documentation for other options you might consider when importing.
Full install times on an Intel 6700K i7 and an Intel Core i9 :

V5.0

Above - Above the clouds. Between the clouds. Among the clouds.
Anime - Hand painted skies inspired by popular animated movies, such as those by Studio Ghibli.
Planet - In orbit of an earth-like planet.
Epic - More generally busy skies.
Mammatus - Thick layers of cloud reminiscent of this unusual cloud type.

Added online documentation & sky catalogue.
Numerous fixes and tweaks. Removed seams. Updated settings. Fixed bugs.
Note : AllSky is a very large asset. It could take hours to import the complete set of skyboxes! It is recommended to only import small subsets of skies at a time.Consult the AllSky catalogue in the documentation to find skies you want to import.

V4.0

AllSky now contains over 200 skies.
Version4 - A whole new set of very high detail skies with a 16k equirect texture.
Space - Some cool new space scenes.
Cartoon - More simple gradients for use as a base if you want to add your own clouds.
HDRIHaven - A special note : HDRI Haven provides Creative Commons Zero (CC0) HDRI photo-sourced equirectangular textures. I have taken those that showed full and interesting skies and extensively edited them (tonemapped, removed the horizon, graded) and including them in Unity usable formats with demo scenes. Note they are not HDR in the AllSky versions. Those can be downloaded for free at https://hdrihaven.com, check them out if you want access to more HDR cubemaps. Support HDRIHaven's patreon if you want to see even more! (now known as PolyHaven)
Misc - A few new ones here and there and fixes to old skies.

Previously all skies were provided in the 6-sided format. Now 99% of skies have their equirectangular equivalent in the folder too.

V3.0

Epic - A selection of absolutely vast and detailed sunsets, storms and cloud formations.
Haze - The sun is out but the atmosphere is dense and foggy.
Sunless & Moonless - Fulfilled frequent request for skies which have no distinct sun or moon position. These have no sun or moon glare or object on them and clouds which don't portray any distinct illumination direction. They are also pleasingly reminiscent of Sega arcade games - 'Blue Sky Gaming'.
Space - Two new space skyboxes. These are fairly dark cloud nebulae and have no stars baked onto them so you can add non-texture resolution dependent stars via your own means (such as particles, shaders or object billboards).

V2.0

Fantasy skies at different times of day with very dramatic clouds and colours.
Cartoon skies in distinctive painterly, watercolour, airbrushed or pixelated styles.
Space skyboxes with epic, colourful nebulae.
More overcast skies for moody scenes.

All 140 skies now have a low poly demo environment with example lighting and fog pass.

AllSky - 220+ Sky / Skybox Set