I just found a very interesting post by Andrew Hansen where he says that your blog’s page rank and spiderability could suffer if commenters leave broken links in their comments ( see:
Untitled Link). This could also happen after a while if commenters pull or move their websites.
A commenter also pointed out that INcoming links from "bad neighborhoods" should not be able to affect your pagerank as you have no control over who is linking to you, but OUTgoing links (e.g. left by commenters to deliberately harm you) could.
So if this is true, then here is my suggestion for a plugin that
a) checks the URL a commenter leaves if it is going to a live page and if not, requests from the commenter to check for misspellings
b) automatically monitors links if they are live (like XENU does) and sends a report about the link status (e.g. once a month or once a week or so, like a Cron job) so you can immediately fix it.
c) keeps a blacklist of pre-populated "bad neighborhoods" where it checks commenters links against and refuses the URL/comment if it pointing to any site on this list.
This is different than other anti-spam plugins and I at least have not come across anything like this. How about you?