Freemind is a free and open-source (GNU GPL V2+) mind mapping program. It is written in Java and is, therefore, highly cross-platform. Mind mapping can be used to enhance personal productivity by facilitating brainstorming, planning, "brain dumps", and even managing one's to-do list.
On Personal Productivity, questions about Freemind should relate to how to use it to enhance one's personal productivity. If you have more general questions about the program, consider posting on Super User.