A free, hidden link database in the Bing Webmaster Tools
Say what you want about Bing but they have really taken the webmaster tools to another level.
The end of crawling and the beginning of API indexing
Google became the most successful startup in history by crawling the web, building an index of web pages, and ranking them based on popularity. Now I see signs for a potential paradigm shift from crawling to indexing APIs. In the future, search engines might not come out to get content. Webmasters might bring it to them.