ReminderFox is an extension for Firefox and Thunderbird that displays and manages lists of date-based reminders and ToDo’s. ReminderFox does not seek to be a full-fledged calendar system. In fact, the target audience is anybody that simply wants to remember important dates (birthdays, anniversaries, etc) without having to run a fat calendar application.

Download it at http://reminderfox.mozdev.org/

Right-clicking on the ribbon will bring up a list of available context menu options. From here you can go to the ReminderFox options as well as a number of other operations.

Images: Reminder Fox