Collaborative planning software helps people plan projects and activities together on the peer base. Everyone can equally contribute, assign tasks and track the progress. Collaborative planning software is a mixture of project management software, groupware and collaborative software. Tools such as Wrike enable users to organize projects and activities as project management tools and discuss and work on. The added value of such software is that it allows you to decentralize control and responsibility for overal plans and permit online access to plans that is equal for all related participants. (Wikipedia, retrieved 10:43, 14 May 2007 (MEST)).
Typically, collaborative planning software are web 2.0 applications.
(none really tested, but all of these do seem to have their followers).