On the website of Marc Lorenz you can get 3 high quality time-lapse skies (each video in 1080p resolution and 4 different color grading flavors).
These time-lapse skies are great for usage as background in archviz projects.
All footage is creative commons licensed and free for personal or commercial use.
On the website of HDR Mill you can get 4 free HDRI´s.
The free images are available in 3000x1500 resolution and include two nature, one city and one interior scene.