Freeplane is a free and open source application for organising, visualising and sharing information.
The program is a redesigned version of FreeMind, and can similarly be used to create mindmaps, although with more features and functionality.
You can use Freeplane to build regular, static mindmaps, simple charts and diagrams which structure information. At its simplest you just press Ins to add a node; customise it, with an image, text, or icon; repeat the process to add other nodes, then link and organise them as you like.
Freeplane goes further, though. This starts with smart tools to help you build better maps. So for instance conditional styling allows the program to automatically give nodes your preferred look and feel according to their hierarchical level in the map, as well as their content.
That content can be very active. You can hide information within a node, present it on demand or automatically, organise it in folding branches, and use hyperlinks everywhere to help users navigate.
Built-in calendar and reminder tools are useful for time and project management.
There's DES encryption of individual nodes or the entire map.
The program can be extended with scripts and some interesting addons.
And once you've finished, the map can be shared as an image, an HTML page, via Flash and more. (In theory, anyway: from what we saw, the exported maps didn't always bear much relation to the original.)
Freeplane is a capable mind mapping tool. The interface is occasionally quirky, though, and there's a lot to learn, so be prepared to invest quite some time before you're producing anything useful.