Efficient task management is an essential component of community workflow management, all the more as standardized organizational structures and procedures for coordinating activities are often lacking in collaborative communities. Before starting with group task management, first the task management for individuals ("to do lists") needs to be taken care of. Countless task management tools, planner web sites, Personal Information Managers etc. are available. However, task management tool support is not enough. Efficient task management requires some form of task management methodology.
Having tried many approaches, I finally chose David Allen's Getting Things Done methodology. It offers the right mix of comprehensive, yet flexible procedures for collecting, processing, organizing, reviewing, and using to do-items.
As to the tools that best support this methodology, I have settled down on the following two, MonkeyGTD and Remember The Milk.

MonkeyGTD is a tool specifically tailored to the GTD methodology. The second version (MonkeyGTD2.1 alpha) is much more powerful than version 1, yet robust enough to be actually used in daily practice. MonkeyGTD itself is built on top of TiddlyWiki, which is characterized as a "reusable non-linear personal web notebook". One powerful feature of MonkeyGTD (TiddlyWiki) is that it is just a simple html-file which can be read with a Firefox browser. No other software is needed. Another, very useful characteristic is that it is based on the principle of "tiddlers" which can be very easily cross-referenced and searched. Some disadvantages are that it can only be read by Firefox, the file can become very big over time and does not allow for easy separation of data and code (cumbersome with upgrades or reorganization of data), and that it is not a server-based solution, so that file management and synchronization can become tricky.

Recently, I have switched back to Remember The Milk, as it deals with many of the disadvantages of MonkeyGTD. It now also has an offline version, which is automatically synced when back online. Furthermore, it allows for very powerful searches based on combinations of tag properties, which can be saved as lists, which are automatically updated from then on (so-called "smart lists"). However, a disadvantage of Remember The Milk, compared to MonkeyGTD, is that it is not dedicated to the GTD methodology. To this purpose, the user has to define and implement a procedure herself. This is not a trivial task, and many different ways of implementing GTD have been proposed, see for example this discussion.
Here is my own procedure, which seems to work and scale fine for me:
This tool plus procedure is the (tentative) result of a many-year personal quest for the holy grail of task management support. It will not be the best solution for everybody (probably not even for myself), but these tips should at least give you some ideas on which to build.