ThoughtStream is a Palm program for storing & organizing linked notes. Windows and unix versions based on the WxPython toolkit are in progress. It is under the GNU GPL. It was originally called synapse and shares some concepts with TheBrain.
Home page - http://thoughtstream.org/ThoughtStream pretends to be a program to write ideas the way you think. Just it has a terrible interface that isn't intuitive at all. The buttons have bad labels, and many menu items doesn't have a obvious effect. Maybe with small details it could seem more like TheBrain (which doesn't have a palm version). Mainly, it doesn't have the click and drag way for easily posting new ideas...
There is another project that tries to emulate TheBrain in a PalmOs. It is called the PersonalBrainOnPalm, but it's very complicated to install and doesn't seem to have an intuitive interface. 'This last comment was made by someone (me) that wasn't actually able to install the program in its PalmTop(HandSpringVisor?). If you have done it and have something better to say, please erase this.'
See also: PalmWiki and WikiLikeThing.