There are some useful comments about bubble.us here: Mind Mapping Software – Fast, Free, Simple, Online.
The reviewer Dane Morgan says “But most of all I like that the maps it produces look like mind maps look in my mind.” I would add that it’s also able to produce concept maps of a sort – a network rather than a hierarchy.
I’ve added the link to the bubbl.us review entry in the database and it will appear on the site soon.
Vic
Updated – typo correction