Windows/Mac OS X only: The robust mind mapping and brainstorming application from ConceptDraw—normally a whopping $US200—is free for a limited time. The app is packed with features, making it a great standalone, but it also plays nice with Microsoft Office apps. If our guide to mind mapping piqued your interest but you weren't keen on the tools we used, ConceptDraw's excellent solution is worth a download. I'm having a tough time determining whether this offer is for the both the Mac and Windows version or just one, but I'll update when I get to the bottom of it. Update: The free licence key works on both Windows and Mac versions of ConceptDraw MindMap, but there's a trick to it.
The auto-generated licence key only works with the standalone version of ConceptDraw MindMap 5, and the version currently hosted on the ConceptDraw web site is version 6 (you also can't download MindMap as a standalone from their site). If you don't mind being a version behind, you can download the Windows version here and the OS X version here. I tested on both platforms, and the key work on both.