Darren Rowse over at ProBlogger recently reviewed two writing tools for the Mac that both sound pretty nifty: DEVONthink Pro and Scrivener. (Darren’s review is aimed at bloggers, of course, but it’s easy enough to read between the lines.)
DEVONthink is a database of sorts, but one that allows you to import text from a range of sources, and then export or print what you’ve put together. Scrivener features a virtual corkboard, and looks to be geared more towards brainstorming.
I can think of lots of potential GMing applications for these two tools, from writing adventures to building worlds and planning campaigns. Both offer free trials (30 days for Scrivener, 150 hours for DEVONthink), and both are Mac-only.