I did have Freemind for awhile. Whilst it's free it simply doesn't have the functionality of MindManager which links seamlessly to Office which is great for transfering mind-mapping ideas to eventual Word documents (or vice-versa). A particularly handy tool is that you can set up templates to transfer your map across to Word, including examining your map hierarchy and making automatic structured headings. Under each map item, you can add a set of notes. If you export your notes with your map, the text is pasted under the respective heading in Word. Why is this useful? Well, it means you can separate your writing stage from your ideas stage, drag boxes around the map to order and re-order your thesis sections. When you've got your structure as you want it, you export the lot to Word and it creates the chapter.
There is also a function called Brainstorming in which you just let fly with all your ideas and then organise the boxes later. It can also be used to set up tasks with start and end dates.If you integrate it with Outlook diary it tells you what you should be doing at any given time.
Sorry if I sound like a salesman for MindManager, but it's been an absolute god send for organising my thesis and my ideas. Mine cost about £50-ish for a student licence, but you can have a month's free trial and decide whether it's for you. If you do try it, try all of its features (I'm still discovering new ones) so you can see what it can really do.