Freemind is a free, open-source application available for Mac OS X, Windows, and Linux.
You can find Freemind here.
While Mindmeister is an online system and allows for easy collaboration, Freemind is a desktop application and does not offer collaboration features. However, Freemind is very capable, and does have the ability to export mindmaps to a variety of formats including word processor outlines and several web page formats.