Mindjet MindManager is one of my most frequently used applications. I use it to brainstorm, conduct meetings, take notes, gather my thoughts for writing projects, and even to plan trips. MindManager has always been my favorite non-Microsoft application ot demo the great flexibility and utility of the Tablet PC as well.
MindManager was also just named as one of PC World magazine's 100 Best Products of the Year.
Not a bad day's work.
UPDATE: Rob Bushway at GottaBeMobile adds two comments of interest. He points out that files can be exchanged cross-platform with no other issues. He also notes that if you use a Tablet PC and have ink in your maps, you need to convert it to standard text before trying to view the map on a Mac.