The Pomodoro Timer
The Pomodoro Timer keeps you on a study schedule of 25 minutes of focus, and 5 minutes of break. Studies have shown that unbroken hyper-focus used like this is massively helpful for your productivity, and I've found it incredibly useful in my studies.
The Timer displays the amount of time left to go ham (out of each 25 minute session), as well as the total amount of time left above it.
[There was a UI picture here previously. I promise I'll update it eventually.]
The smaller gray timer tells you how much time is left in your total study session, and the larger blue timer tells you how much time there is until your next mini-break.
You can dynamically add time to your study session at any time with the buttons on the right side; use them to start a new session or extend your current session.