Probably nothing you haven't heard of unless you'd be happy compiling and using an incomplete "project" browser, like Ladybird. The list with pre-built binaries includes: Mozilla Firefox (or Waterfox), Google Chromium (or Brave), and Apple WebKit via Safari (so, open source engine, closed-source chrome) or other browsers that use WebKit (e.g. DuckDuckGo's Orion).
Did you mean Kagi's Orion?
Thank you! Yes, I was confused — DDG's browser is "DuckDuckGo for Mac", and also uses WebKit.
Safari's rendering engine is open source, but the UI isn't.