Mind Mapping software PersonalBrain is cross platform and available on Mac, Windows and Linux. Mindjet Mind Manager is available for Mac and Windows.

So should you buy Mind Manager on Mac or PersonalBrain ? MacWorld has this advice:

While there are plenty of outliners, Getting Things Done, and project management tools for the Mac, PersonalBrain 4.5 gives you a powerful, innovative non-linear capability. Its closest competitor, MindJet’s MindManager 7, has a more Mac-like interface but imposes a great deal more structure on your mind maps.

Personal Brain: Creates intuitive 3-D mind maps; takes a non-linear, context-sensitive approach. Steep learning curve for unfamiliar interface; no way to import existing OPML outlines; expensive.

MindManager: Outline view allows you to easily switch between different organizing principles; many new export formats; easy to move data to and from outline and project-management applications; better printing options. No Microsoft Office integration; maps exported to and from other applications lose special formatting.