It is freeware and open source. There is already a portable version, but it is not set up in the PortableApps format. I'm too klutzy to do it myself. Anyone want to take it on? It's a very useful program. Available here in both full and portable versions: http://www.xmind.net/
As best as I can tell, all you need to do is :
1. Download the xmind portable version
2. Copy the windows and Commons into an xmind folder in your portable apps folder.
3. Edit the xmind config file and change the ../Common references to ./Common (IE, it'll access the same folder in a different location).
4. Refresh your apps, it should be there, and it should run fine.
-Ray
FYI - this works.