With so many web applications and sites in the current time, it's hard for a user to keep track of where does her personal information reside. With Appu, we aim to make this job easier for the end user. Appu keeps track of personal information such as passwords, username, birthdate, address, credit card numbers, and social security number so that a user can find out all sites that store a particular bit of personal information.
In the current beta release, Appu downloads personal information from sites where you have account and also tries to prevent password reuse across websites by warning users about it. You can read about why reusing passwords is bad here.
Nick has written an excellent blogpost about this tool here.