IOU is a debt manager app developed by Sergey Tubin for iPhone users. Its main purpose is to keep track of loans, periodic payments, bills, and personal debts all in one app. With IOU, users can borrow or lend things and money, make partial payments, and view the history of settled debts.

The app also has several useful features such as notifications for overdue debts, recurring debts and payments, password or Touch ID protection, and cloud sync. Users can even share their debts with their contacts and attach photos to their debts. Additionally, IOU allows users to send email or SMS reminders to debtors, and automatic email reminders can be set up with syncing.

One of the most significant advantages of IOU is that it syncs to ioutool.net, allowing users to keep and edit their data on different devices and in a web browser. Overall, IOU is an excellent app for those who want to keep track of their debts and payments conveniently.