Ok, here's a trick of the trade I've been using for years.
- Search web directories for web sites which no longer resolve
- When you find one, check if the domain is available or not
- If it has expired/is available, use OpenSiteExplorer.org or something similer to check back links
- Then use Archive.org's Way Back Machine to check how long it has been a live website in the past and rip off any old content from its pages.
- Recreate the site on a something like WordPress, with any original content from the Way Back Machine - make sure you reuse all the old URLs to ensure you capture any other link juice.
- Wait for Google to re-index.
- Enjoy some nice PR!
The beauty of this is that if it was an old site it will regain trust straight away after being indexed, much better and effectively only costs you some time and the reg fee...
I have my own php script which I built to do the first 3 automatically for me. I can input the URL of a page within a directory (or any site for that matter) the script checks the status code of all outgoing links on that page. If it finds any links which return 400 or 500 codes it then queries the whois to see if it is registered or not. If not it queries a link checker API and confirms authority/PR of domain (if any).
Giving away quite a bit here, but feeling in a somewhat generous mood.
Hope this helps
Best
Gary