Mindmeister allows users to participate in real-time collaboration in global brainstorming sessions on the web. Users can be anywhere and work together anytime; seeing each other's changes as they are made. Skype calls allows the invited participants to discuss new ideas and put them into the mind map at the same time.
As you share your mind maps with friends or colleagues, the invitees will receive an email with a link and will be able to add their own ideas and comments. Old versions of the mind map are saved on the MindMeister server so you can replace the new ideas with a previous version if necessary.
I really like the brainstorming mode (two or more users are in the same mind map at the same time) and you can see every change that is made instantly through color-coded effects. Working offline is an option through Google Gears and when you go back online, just slide the button to online and all the changes will be automatically synched.