Having used the Arc browser on my Mac for over 7 months I can't imagine going back to Safari. The 'spaces' paradigm and vertical sidebar work so much better for me.
As I watch their video updates on YouTube the algorithm did it's thing recently and suggested a video comparing Arc with SigmaOS, another browser which I was previously unaware of despite it being available longer than Arc.
I was intrigued so installed it. The two are conceptually quite similar, both working on the sidebar with spaces idea, although SigmaOS uses the webkit engine while still allowing you to use Chrome extensions.
SigmaOS treats the web as a to-do list, you 'lock' tabs that you want to keep around and mark those you're finished with as 'done' just by pressing D. In fact, one of Sigma's main selling points is their novel approach to keyboard shortcuts: dispensing with a lot of standard combinations in favour of single keys where possible.
I sort of like it and think if I had known about this before Arc then it might have become my browser of choice but it feels a little clunkier. You also have to pay a monthly subscription to get more than 3 spaces and cross-device sync whereas The Browser Company have said that all core browser features will remain free.
I had also heard of the Sidekick browser but never seen it – it looks interesting with a focus on productivity through the integration of web apps (again in a vertical sidebar) but it's not really for me.
While digging I also found Orion, also based on webkit, a very similar appearance to Safari but almost what Safari should have become. I say almost as it's a little clunky in places. Again, Orion allows you to show tabs in a vertical sidebar rather than just Safari's tab grouping.
It's great that we are getting differentiation in the market and that different browsers can appeal specifically to different types of users with varying requirements. Personally, Arc sits best with me.
