I use XMind. I think it's a great piece of software. They do a free version and a paid for Pro version, however I find the free version has most of the stuff you'd want anyway.
For me the best thing it does it allows you to drill down into specific nodes. When you drill into a node that node then appears as if it were the root node. You see all it's children but not it's parents. You can obviously drill make up to see the bigger picture.
I find this very useful as my maps often get quite big and it allows me to focus on a specific areas easily.
It comes in both mac and windows downloads, give it a try.