The Google alert tool (http://www.googlealert.com/) keeps a watch on new entries within the top 50 on Google for one of up to 5 keywords or/and a total of 150 query requests. How it works is it will email you every time a new entry enters the top xx (where xx is the number of results you wish to test) for a chosen keyword. So I have used for example as a related term to my website the keyword 'web design'.
Each time a new site enters the top 50 with that search term I get an email with the domain and the exact rank. The webmaster of this website makes for a great potential link partner.
Link Trading 
