I've just been digging in someone's Google Webmaster Tools account, and discovered some gold nuggets in the error reports. The reports identify hundreds of old links on external sites pointing to URLs that no longer exist on his site. Some individual URLs had dozens of links.
So, I've compiled a list of the old broken URLs, matched them with suitable new URLs, and requested that his web team set up 301 redirects. This will give users a better experience, and of course I'm keen to salvage whatever Page Rank can be gained here, too. The links are mostly "little" ones, but there are lots of them.
ACTION STEPS: Log into Google Webmaster Tools for the site, and go to the "Not Found" Crawl Errors report. At the bottom of the page, there's a link to "Download all sources of errors on this site."
That gives you a .csv file which lists all the problem URLs Google has found, and also lists the source of each error. If your site is young, your report might have few entries, but if it has been around for a while there might be hundreds.
If any of the errors are coming from within your own site, or a source within your control, make it a priority to locate and update those broken links right away. Broken links waste Page Rank, and won't help your signals of quality, either.
Then, gather the URLs that have links coming in from external sites, weed out any duplicates, and decide on new URLs to redirect them to. Set up 301 redirects from the old to the new, taking care to avoid chains of redirects; make the redirect be one step only.
There's no way to measure the value of Page Rank that could be salvaged this way, but it's better to have it than not, right? :)