Here’s an interesting web-based collaboration tool that has some of the characteristics of mind mapping software: Thinkature. This tool features a virtual "whiteboard" where you can add topics, move them around the workspace and join them together to show relationships between ideas. Anyone you’re collaborating with can create and manipulate objects in the workspace, and you can even have a live chat with your colleagues while working together on a set of ideas. Cool!
At this point in time, Thinkature is free. You can sign up for an account and begin collaborating in a matter of minutes.