The Six Revisions Weblog recently published an article reviewing 15 free tools to help you facilitate remote/web-based collaboration which, although geared towards project management, could easily be applied to planning activities. The tools cover a range of applications from basic whiteboarding, mind-mapping, participatory editing, brainstorming and remote conferencing tools, right through to  fully-featured project management applications.

See the article here.

Wikipedia provides a series of useful discussions of online collaborative planning software tools  as well as a more in-depth review of collaborative software in general.