The best tool ever for me, especially because I use emacs, is the planner mode. http://sacha.free.net.ph/notebook/wiki/PlannerMode.php
There is a "multi" option within. This option allow you to have a task in different projects/contexts, quite nice for GTD setup. I am more using it to make a list of things todo in the current week. Then every morning I check my week page and schedule items in my day page.
I have tried many others, web based or not, personal hacks or not, and was only pleased after using this mode. If you are using another text editor, I am sure you can find a similar tool to manage your todo and calendar from within it.