Inspiration 7.6 may be in a class of one. This relatively obscure (educational market) outliner/mindmapping/visualization software runs on OS X, Mac Classic, Windows (any version), PalmOS, and PocketPC. The Palm and PocketPC versions include conduits. File formats are the same on Mac and PC. The Mac version used to be able to read MORE 3.1 files (Omni Outliner does that better now.)
Only FileMaker 7 had comparable coverage (the Palm and Pocket PC versions of FM 8 are mysteriously delayed.).
I've used Inspiration occasionally over the years, but I'm going to try it again on my Palm. (Note the CD ships both Mac and Windows versions, and, for better or worse, they use the same key.)