• Achoo HTML Viewer & Inspector
• Amplosion: Redirect AMP Links
• Baking Soda / Vinegar: Replace custom videos with native HTML5 video tags (Vinegar=YouTube, Baking Soda=everywhere else)
• History Book - Browse & Search: Every page you visit for more than x seconds gets added with its contents to a full-text index so you can search for that sentence you remember but don’t remember where you saw it
• Noir - Dark Mode for Safari: Works better than Dark Reader for me
• Redirect Web for Safari: Custom redirect rules, e.g. replaces Mapper and Customize Search Engine (to redirect to my own SearXNG) for me; also use this to redirect to Facebook’s chronological feed every time I open the front page
• Rewinder - Time Travel the Web: gorgeous Wayback Machine UI, albeit very slow b/c archive.org is slow
• SingleFile for Safari: Latest addition, not used it much; allows to save a complete web page incl. all assets to a single file similar to MHTML
• SponsorBlock for YouTube
• Table of contents - for Safari: Provides a TOC for web pages to easily jump to different headings
• Wipr: Pay once, install and forget ad-blocker. Works very well for me - incl. YouTube ads.

Not a Safari extension, but related:

• Opener - open links in apps: Share a deep-link to this tool and it provides a list of apps to open the link in