I came across Planner mode a couple of weeks ago. It has been working very well for me. It integrates with a variety of other EMACS applications. It is also able to keep track of time spent on a given task, which is handy for a number of uses.
In the past I have used a variety of contact management/calendaring software including Outlook and Evolution but they are big and bulky and get in the way. They are also overly complicated and it is hard to associate notes with a given piece of information.