Calendar applications can sometimes be overly complex. If you’re looking for a Firefox or Thunderbird extension that gives you good, but not over-the-top reminder functionality and a nice interface, ReminderFox may be what you’re looking for.
It offers date based reminders for appointments, birthdays etc with pop-up and snooze options, as well as a handy ToDo list. I’ve had a go at several different calendar/reminder apps but this one is definitely doing the biz.
Download the latest version here.