Freemind is a fantastic mind mapping program. It is free and open source. I created a small document icon (.ico) for Windows users to associate their Freemind documents (.mm) with.
To associate a specifi icon with a document type you open a folder, go to menu Tools :: Folder Options… :: File Associations and change the file type for the extension .mm.
You are welcome to download it from the link below. 🙂