ReminderFox

ReminderFox is an efficient extension for Firefox that lets you note your important dates and tasks on a calendar without having to use another application.

You can find the extension on the status bar in your browser, and access it by clicking on its icon or with a right-click. You can change the icon location, too.

This plugin lets you keep all your meetings, dates, and events under control. You can set up reminders to the exact minute, and also add a short description to every entry so you don't forget any important details.

Use this program to manage your to-do list as well, and once you complete your tasks you can cross them off the list.

ReminderFox is completely customizable. It even lets you customize the sound of the events reminders from the settings panel.