Search Engines look at the web like a large voting system. The more links there are to a site, the more people are voting a site as something of interest, a destination.
There are many free tools that enable us to analyse sites to see who is linking to them - the Google Toolbar has a 'Backward Links' button that will show Google's analysis of a particular page but Optilink provides more detailed analysis from a number of search engines including AOL, AlltheWeb, Altavista, Google, Hotbot, Inktomi, MSN & Yahoo. You can request up to 1000 links at a time & you can see the anchor text that webmasters are linking to the site with.
I've enclosed a link to the video of the software which details the application in 2 minutes: http://www.windrosesoftware.com/optilink/video/ol-qs.html
