MSN has a very cool new feature described at SEOMoz.org that allows you to see whose linking to your competitors but not to you.
For example a search for:
(linkdomain:amazon.com.com linkdomain:bn.com) (-linkdomain:borders.com)
Will provide a list of all sites linking to Amazon.com and BN.com but not to Borders…
This is a great way to find potential link partners, as well as to find areas of the web where your visibility is low compared to your competitors.