Friend or Follow: Find out Who Follows You Back
Friend or Follow is a free online tool which can be used for finding out which Twitter users follow you back, and also who you are following back out of the people who follow you.
Simply type in your user name and press submit, and then hover the mouse over people's avatars to view their Twitter bio when it returns the results.
The tool gives you the option to sort the results in a variety different ways, including by date of a user's last Tweet, which is useful for weeding out old and inactive accounts.
It also gives you the option to tweet a list of the people who aren't following you, although personally I'm not sure I see the point of that particular feature myself as if somebody isn't following you, then they aren't exactly likely to see it!
In summary, a useful tool and worth checking out if you haven't seen it before.
Twitter isn't meant to be a popularity contest - the number of followers you have is less important than the quality of interaction that you have with people. Just because somebody doesn't follow you, it doesn't necessary mean that they don't add value to your PLN (Personal Learning Network) if they engage with you in discussions.