WebWorkerDaily has covered many time management, to-do list, and calendaring applications such as RememberTheMilk and Toodledo. There are many good applications in this space, but the tricky part with many of them is that they are intensely personal, and the way they work may not suit the way you work.
For a very simple, unobtrusive solution that works directly within the Firefox browser, ReminderFox is worth a try.
ReminderFox is an extension for Firefox that primarily focuses on tracking your important tasks and dates and sends you alerts or alarms about them. It doesn’t aim to be a full-fledged calendar application, and for that I find Google Calendar to be the tool of choice, but it provides a very quick-in, quick-out way to enter and track milestones. You can view your collections of to-do items and reminders in list format.
When you receive an alert or alarm in ReminderFox you can choose Acknowledge, Mark as Complete, Delete or Open to see more information about the task. You can also hit a Snooze button to make a reminder go away and come back at a set time interval.
While ReminderFox is not calendar-centric it does provide a rudimentary calendar. You can view to-do list items and reminders in conjunction with the calendar, which is useful for making plans.
For these kinds of applications, my main requirements are that they are quick to get in and out of, and simple to use on the fly. ReminderFox meets these requirements, even if it doesn’t shoot for some of the bells and whistles found in applications that reside outside of your browser.