This page describes the FreeMind file format (mm), which is reportedly used by several other mind mapping software programs (at least for import and/or export). The file format can be created, read and manipulated using the FreeMind Open Source software application. Tools and mechanisms for import and export of maps and conversion to other formats are listed.
The format is referenced in the Wikipedia page on mind mapping software. The XML schema for version 0.9.0 beta 2 (the latest "production version as of November 2012) is at http://freemind.cvs.sourceforge.net/viewvc/freemind/freemind/freemind.xsd?pathrev=fm_0_9_0_beta2&revision=188.8.131.52. The schema for the latest development version as of November 2012 is identical.
The file extension is .mm and the MIME Type is application/freemind or application/x-freemind.