While trying to look-up a shortcut for man-pages in What shortcuts can I use for linking to other information?, I noticed that there is no such thing as a [man://] tag. Although, I know that there are various flavours of Unix, it would be convenient to point to a site such as http://www.freebsd.org/cgi/man.cgi for further reading. E.g. [man://env] ⇒ env could be linked to http://www.freebsd.org/cgi/man.cgi?query=env, which will present a default man-page for FreeBSD. There, the reader can select the same man-page for another Unix flavour if desired.

Potential variations could include (with env as an example):

[man://env] ⇒ env

look-up 'env' for any Unix (FreeBSD by default)
(→ http://www.freebsd.org/cgi/man.cgi?query=env)

[man://env(1)] ⇒ env(1)

look-up 'env' in section (1) / note the CGI NVP: sektion=1 (not(!) section)
(→ http://www.freebsd.org/cgi/man.cgi?query=env&sektion=1)

[man1://env] ⇒ env(1)

same: look-up 'env' in section (1) / analogue: man2, man3, etc.

[man1://env|env (1) manpage] ⇒ env (1) manpage

same but user defined text

[man://env(SunOS 5.10)] ⇒ env (SunOS 5.10)

specification of a distinct OS - could be challenging to implement reliably / note the CGI NVP: manpath=SunOS+5.10
(→ http://www.freebsd.org/cgi/man.cgi?query=env&manpath=SunOS+5.10)

[apropos://env] ⇒ (apropos) env

don't know if that makes sense, but if [man://] is possible, [apropos://] is easy / note the CGI NVP: apropos=1
(→ http://www.freebsd.org/cgi/man.cgi?query=env&apropos=1)

At least the [man://] and [man1://] [man2://] ... variants should be straightforward to implement.
The freebsd service seems to exist for several years, so there is a little bit of confidence that it will be available in the future.

For Windows related references to the KB MS Knowledge Base, the [mskb://] tag could be useful. E.g. [mskb://245225] ⇒ MSKB 245225 being a link to http://support.microsoft.com/kb/245225. Analogue: [mskb://245225|HOW TO: Configure and Test a PERL Script with IIS 4.0, 5.0, 5.1, and 6.0] ⇒ HOW TO: Configure and Test a PERL Script with IIS 4.0, 5.0, 5.1, and 6.0.

I am not sure if linking to those sites might have some legal implications...

Update: NVP = Name/Value Pair, nowadays Attribute/Value Pair
Update2: samples highlighting the differences added; links for CGI, NVP, and KB added.
Update3: Summary and update here.
Update4: Today, 16-mar-2009, the [man://] shortcut has become reality! Thanks to pmdev - especially jdporter - for implementing this suggestion.