I like the idea. I know some portion of my reputation comes from answering exactly this kind of question but I think the Internet overall could be improved with this information.

The list of URLs submitted here ought to be curated site-wide through some mechanism. e.g., if I paste in a linux.die.net manpage link, it'd be nice to have back-up manpage links available if linux.die.net goes down. Having a single list of all URLs used as general references could make finding dead URLs far easier as well as allow easier standardization.

Further, it'd be nice to provide multiple links to be shown on questions: I love using both the POSIX and distribution-specific manpages when both are available. What is standard and what is implemented are sometimes two different things, and having both sets of details within clicking distance of each other is immensely useful.