Keyworx

is actually two trackers in one. A keyword tracker that reverse engineers rankings to let you see exactly what is worked for you, as well as a BSR tracker that allows you to track your performance over time for each ASIN.
 
KeyWorx stands out for me because of the balance between affordability and function. KeyWorx delivers a great service that is backed up by some great tutorials from a great team, all with a very affordable price tag.