Since February-2015 ThinkComposer is an open-source product, licensed under the GNU General Public License 3.0.
This roughly means that you can freely download, share, modify or extend it, as long you provide also a free copy of the original source code plus your own modifications and extensions.
The source code is available on CodePlex at: http://thinkcomposer.codeplex.com/
Recently, ThinkComposer was reviewed by Roy Grubb of Mind-Mapping.org. He did a great job summarizing the product and giving feedback of both, its pros and cons, plus in relation with other products.
Also, ThinkComposer was included in the very extensive catalog (the “master list”) of visual tools that mind-mapping.org exposes, which is a great source of information of the market tools and their way of help visual thinkers. Great!