None that really come to mind, unfortunately. There was one called Fanbrowser that used to do this briefly a very long time ago, but that project has been torn

None that really come to mind, unfortunately. There was one called Fanbrowser that used to do this briefly a very long time ago, but that project has been torn down and rebuilt from scratch so many times that none of that functionality is left. One iteration of it let you browse through a directory of interests (sourced from the DMOZ). When you found a site, or if you typed in a URL, it would load that page in an iframe. You could bookmark pages and leave comments which other users would be able to see. Unfortunately, it didn't get much further than that, so it never had things like being able to follow other users or anything else. Plus, iframes became increasingly difficult to work with since web browsers started adopting a lot of privacy and security measures like CORS. But for a brief while, it was fun.