Mind mapping software won’t actually start breeding ideas for your essays on its own, but it will certainly help you to organise and structure your thoughts, and many other useful things besides.
FreeMind is very highly thought of, because of its ease of use, and its ability to deal with many types of information input. Give it a go! It’s Free!
Here’s the FreeMind wiki, and there’s a good explanatory video to download here. Here’s the all important download link. Because it’s written in Java, it will run on Apple, Linux and Windows. (Check that download link for details).
If you prefer to share your mind maps with a group online, have a look at the web based MindMeister.