Because I was just looking this up the other day, here is my list of the remaining Unixes that are still alive and where to find their manpages online. Of these, the ones that I think matter significantly any more as Unixes are Linux, FreeBSD, OpenBSD, Solaris (maybe not for much longer) and perhaps MacOS X (depending on what you're doing).

The list:

• Linux (yes, really)
  It's a bit hard to find current Linux manpages online, although there are lots of places with somewhat out of date ones available. The man-pages project is probably the authoritative reference for system calls and glibc stuff; for manpages in general, see tldp.org's manpages section and its collection of links, especially this searchable version.

  (There's also man.he.net.)

• the *BSDs: FreeBSD, NetBSD, OpenBSD, and Dragonfly BSD.
  All of them have manpages easily available through the documentation section of their own websites; FreeBSD manpages, NetBSD manpages, OpenBSD manpages, and DragonflyBSD manpages.

• Sun's Solaris, oops, I mean Oracle's Solaris
  At least until Oracle reorganizes everything, the Solaris 10 manpages are part of the Solaris 10 documentation collection, and can be found online here.

• Apple's MacOS X
  Online manpages are currently here, I would assume for the current version only.

• IBM's AIX
  The manpages for AIX 6.1 really need JavaScript, and a few previous versions seem to only be available in PDFs if you know the right 10 Mb PDF that you want.

• HP's HP-UX and perhaps Tru64
  HP seems to have not made up their mind about where their manpages are; online manpages for all sorts of HP-UX releases are here or less usefully here in PDF form. There's also Tru64 stuff here, all in PDF.

Sadly departed is SGI's IRIX, among many other Unixes.

For a general reference on portability, see the Single Unix Specification (background here).

(By the way, my criteria for whether an online Linux manpage collection is sufficiently up to date is whether its ipv6 manpage mentions IPV6_V6ONLY, since this is what I was looking up the other day.)