http://freemind.sourceforge.net/wiki/index.php/Main_Page A free download.
http://www.imindmap.com/ Endorsed by Tony Buzan. A free trial, or purchase. There are also a number of other resources on this site.
http://www.inspiration.com/Inspiration A free trial version can be downloaded. Also: Kidspiration, and InspireData, for data visualisation. This concept map on the front page of this resource was created with Inspiration. The site also has information about Kidspiration and Inspire Data.
A free trial, or purchase. There are also a number of other resources on this site.
An excellent program that supports collaborative knowledge creation. MirandaNet uses this program for collaborative knowledge construction in MirandaMods. MindMeister has two versions: a free one, with a limit on the number of maps that can be stored on the site, and an inexpensive paid-for version.
http://www.innovationgear.com/index.php For purchase. A free download is also available.
http://www.novamind.com/download/ Various versions for purchase.
http://www.solutionlanguage.com/ A free trial, or purchase. There are also a number of other resources on this site.
http://www.visual-mapping.com/ An interesting blog, with resources.