At HDRIHaven.com you can get really good high resolution HDRI images absolutely free.
The site is mantained by Greg Zaal, a CG artist and Open Source advocate.
All HDRIs are CC0 (public domain). No paywalls, accounts or email spam. Just download what you want, and use it.
The site is very clean and simple to use. Textures are well organized by category and size.
Users can support the initiative at Patreon.