The challenge is that the same file structure is used to store the mind maps - <some folder> \ <some mind map>. After creating a few dozen mind maps, I find the brainstorming capabilities start to get bogged down in trying to keep track of the maps.
Has anyone found a tool that might address this challenge, keeping in my mind Sharepoint and other large collaboration tools are too expensive?
I think you have run into a fundamental problem with information management and communication.
The easy bit is creating the content.
The hard part is creating an easily maintained, understood and used library system for that information.
I am not being facetious when I suggest a database might provide a solution. There is a lot of skill in designing the taxonomy, structure and metadata for information artefacts of which mind maps are just one.
I've toyed with the idea of the SQL2008 remote blob store to help build something light and simple but at this stage it is just an idea. I have considered keeping suitable metadata in tables and putting full-text indices over the top of them but I feel that full-text indices are a bit limit in their capability. They don't really allow phonetic searching which would really boost their applicable uses.