TwiPing Pro

Twiping Pro has all the features of TwiPing Basic, but also:

Supports unlimited Twitter accounts
Supports unlimited Follow-Back lists

Follow-Back Lists track the percentage of users that eventually follow you back. In addition, once you are done waiting for follow-backs, they make it easy to unfollow those who didn't follow you back with the click of a button.

NOTE: TwiPing is a Windows application. No Mac support or web application.