Nag

Nag is a simple alarm clock designed for Mac. This utility might prove to be of good use if you're dealing with a busy day and wish to be reminded when it's time to complete a certain assignment.
The application features a user-friendly interface, comes with easy-to-configure settings, and uses minimum CPU resources.

Furthermore, it gives you access to various alarm sounds and other notifications options.

All you need to do in order to set up an alarm is to select the preferred interval of time and the app will let you know when the timer hits zero. You can use the + and - buttons to add or subtract minutes. The utility also lets you enter a description for the alarm.

Once the time's up, the app will be displayed on top of other running programs and let you know it's time to complete the assignment.

I tested various alarms, with different timer settings. The application worked perfectly and played a sound notification as soon as the timer hit zero.

The one thing that bothers is that you cannot set multiple alarms. The iOS version of the program provides you with feature.

Still, you should give this free app a try if the aforementioned aspect doesn't mean much to you.