I recently found out about a nifty tool called HacknPlan, which can be used to plan game development. It's currently in open beta, but it's very feature complete.
Here's a link to the website (it's a web app): http://hacknplan.com/
I'll give a short explanation on how it works: You can create Projects, which can contain Milestones, and which you can add Members (collaborators) to. A milestone is where most of the actual planning is done; you can make one, for example "Beta 4", and then add tasks to different categories, such as Programming, Art, Sound et cetera. Each task has a "cost" (which is defined in the Project settings), and their own description, category, subcategory et cetera.
It's a very good tool, I hope someone finds it useful.