http://listigator.com which was just recently launched (beta) is a
general purpose list management application with emphasis on
collaboration.
Listigator offers some handy GTD-oriented features, including:
* capability to organize list items into categories/contexts
* capability to save snapshots of lists (for archiving or for using as
a base for a new list)
* capability to tag and search your lists
Lists can be defined as private or shared (both for read and edit), and
similarly to tadalist, there is no charge for using the application.