Letting other people contribute at the planning stage is good for team building.
These few Online systems do just that.
A simple "To Do List" is a great way to start any project. The Ta Da Lists from www.37signals.com does all that is required in a simple way.
There are 2 ways to share this list
Here is a list I have made for the work on My Lounge You will see that if I change an item on the list you can be "notified of changes to this list via RSS:" What this means is that you can register a RSS feed that will automatically send you a message.
Also from www.37signals.com is Base Camp a fully featured planing system.
" Basecamp makes it easy to centralize group communication with co-workers and clients.