Ive noticed there is quite some online manpage websites, but every time when I needed to refer to a certain page, its hard to choose, because I had no idea which one would be reliable.
A manual page may have different sources, for example, the sed(1) on Linux and on *BSD, they are apparently from different sources. On GNU/Linux, it will be the GNU Sed; On *BSD, it will be the BSD version, so the contents will be different as well. Also, even Linux distributions may make small changes on special occasions, for instance, gcc(1) on Gentoo, there is a couple of notes added regarding the changes were made by Gentoo developers.
Some sites, I dont have a clue who is providing those pages, and definitely no idea if it would still be accessible tomorrow. I dont want to have a dead link if I can avoid.
Even some of them I dont fear they would disappear someday, but it doesnt seem to be enough for reference. I wish someone would create one not only having the versions, but also having the anchor to specific line like VCS repository web viewing, section anchor is just not enough.
The following is the list, I split them by the systems and different features, or the tables would look squashed.