I love concept maps as a way of explaining a topic. (Here's an example, capturing what I learned at a KaizenConf session.) If you're a visual thinker, you'll definitely want to check this out. If you design UIs, this is also of interest.
The way I use concept maps most often is exploring and explaining a concept to myself. The act of drawing the map sorts my ideas visually, lets me hang new information off logical hooks, and gives me a picture to visualize when I want to recall the info later. (If you've talked with me about F# and watched where I gestured, you've seen that OO ended up on the left side of my map, and functional programming was right of center.)
CmapTools is a free software application that makes drawing maps intuitively easy—better than paper, because you can move concepts after you've fleshed out more of the landscape. And I have a strong preference for paper for brainstorming and thought-capturing, so that's saying something.
I like it not only as a tool, but also as an example of usability. It's so low-friction because you trigger actions (creating a concept, making a connection) right where you're already focused, in the work area, not in some menu at the top of the window. Click-and-drag from an existing concept to create a new concept and connect them; then type, click, type to enter the labels on the connection and the concept. I admire CmapTools for its non-noisy GUI—and of course I love it as a user because I can create a map fast enough to not lose the thread of my thoughts.
The example linked above is cool in another way: It's on a wiki where participants at KaizenConf are creating the conference proceedings. Once you go self-organizing, baby, you'll never look back.