CmapTools is a free application for generating concept maps, charts/ mind maps which represent knowledge models.
A concept map is a diagram which contains various concepts and uses connecting lines (and, sometimes, text) to show the relationship between them. If you've not come across the term before, or you just want to see what the program can do, CmapTools has a concept map about itself which you can view here.
If you look at the example you'll see it's a fairly simple graph, but does offer links to other maps and web pages. Factor this in with CmapTools' ability to save maps to the cloud, and they can become a useful way to share information and collaborate on projects.
If you'd rather keep your ideas to yourself - or, at least, have more control over who gets to see them - they can also be saved locally as images, PDFs, HTML files and more.
Please note, the Download link takes you to the developer's website, where you must register before you're able to download the program.
CmapTools is awkward to use, and its concept maps seem relatively basic when compared to other mind map types. There is some value in its sharing and collaboration tools, though, so if you need to publish your ideas to a wide audience then it could be a good choice.