http://freemind.sourceforge.net/wiki/index.php/Main_Page FreeMind looks like it's a pretty sophisticated piece of software. ( Needs Java 1.4 )
I'm using it at the moment to help summarise my uni notes. It gives you a tree with folding nodes etc. It's pretty simple to use and has heaps of options/icons etc. You can do stuff like add hyperlinks and links to files it looks like but I haven't personally used them.
From memory it's just a few hundred K download. You can export your "mind maps" to html etc too. It's nice.
Tim
