[layer8]

Linux: https://man7.org/linux/man-pages/

Debian: https://manpages.debian.org/

Ubuntu: https://manpages.ubuntu.com/

FreeBSD: https://man.freebsd.org/

OpenBSD: https://man.openbsd.org/

[creatonez]

Archlinux also now has a manpage viewer

https://man.archlinux.org/

Useful if you need to see the manpage of a very recent version of a package.