XMind -- a powerful, Java-based mind mapping and charting application -- has been open-sourced (it's free!) with version 3.0. While it's not my absolute top pick for mind mapping, it ranks up there. I did have a license for the paid version, and I can tell you that it packs plenty of punch. It has all of the extra features I like: boundaries, relationships, summaries, notes, markers, outline view, floating topics and more, as well as intuitive keyboard navigation for rapid-fire brainstorming.
One of my favorite features was the drilldown; pressing F6 focuses in on the current topic, displaying only it and its descendants. Shift-F6 takes you back up. It's similar to MindManager's "Focus" feature, but faster (and more limited in scope). And for those who want (or need) to, just set all of the shapes to underline, turn on multiple branch colors and tapered lines and presto, you've got an honest-to-goodness Tony Buzan-style mind map ... or as close as you'll get without buying Buzan's software (which, despite it being relatively expensive and lacking in system integration, I am regularly tempted to do) or breaking out the colored pencils.
You can also create Org and Fishbone Charts with XMind. It imports FreeMind and MindManager maps as well as Marker Packages, and can export to HTML, image, Marker Package or Text. The HTML export is ... okay. FreeMind actually creates more useful HTML pages, but it works. Overall, it's great stuff, and it's exciting to see it open sourced.
So, how did software that once cost $299 end up being free? It appears to be a change in the business plan, with a subscription model for "XMind Pro" replacing retail sales. I'm a little sketchy on the details, but the Pro version seems to include online collaboration for XMind users, Gantt charts and a Presentation mode (among other features), with a range of subscription prices: $6USD for 1 user/1 month, $400USD for 10 users/1 year, and everything in the middle. It's not necessary to have a subscription, however, to use XMind. Just download a copy (or even the source code) and let the brainstorming begin. Oh, and tell your PC friends, too; like many Java apps, XMind also comes in Windows and Linux flavors.