from http://www.seomoz.org/linkscape/help/sources

    * Dotnetdotcom.org
    * Grub.org/Wikia
    * Page-Store.com
    * Amazon/Alexa’s crawl and internet archive resources
    * Exalead’s commercially available data
    * Gigablast’s commercially available data
    * Yahoo!’s BOSS API and other data sources
    * Microsoft’s Live API and other data sources
    * Google’s API and other data sources
    * Ask.com’s API and other data sources
    * Additional crawls from open source, commercial and academic projects

In my experience, the single most useful feature (main selling point) of the Linkscape tool is that it reports http status codes (for a price) so SEOs can detect 301 redirects, etc. AFAIK, dotnetdotcom.org has the only free, publicly available crawl data which also includes http status codes. Not sure about Exalead and Gigablast but I am pretty sure the other SEs don't release this information. To clarify: I don't have any proof, and things may have changed, but I've read some intelligent speculation (smarter than me) which claims that dotbot/dotnetdotcom.org provides the majority of the data (especially the unique info, like status codes) for the Linkscape tool.