I want to know if there's an application or website that can help me know who are the people that are not following me back, I have 3,549 followers but I'm following 3,612. So I'll like to know who are those users that don't follow back, because it's not easy to go to every URL of every username and know if they are following me or not. I've googled it and the things I found are not available for download anymore.
Look up the XKit extension pack. I don't think it automatically includes the follower checker when you install it, but you can add it with ease. It tells you both whom you follow that doesn't follow you back and anyone who unfollows you after you install it.
EDIT: here's the Chrome extension.
We just extended our Twitter app (Friend or Follow) to allow people to see who's not following them back on Tumblr, and vice versa. You might be interested in it. http://tumblr.friendorfollow.com/
I very much doubt it is possible without using Tumblr's API. Seeing that Tumblr pages this data, it would just not be feasible to create a bot that monitors this list constantly without the API.
Of course you can also just use your own application locally utilizing the following calls
As long as you ensure respect for API Limitations