[1]: https://mindnode.com/assets/images/screens/iphone-1-1136x640...
Online, there's Text2MindMap[1] which is quick and easy to use, and offers a rubber-band like ability to reposition nodes, but in my experience, once past a couple of hundred nodes (I do some fairly large maps) performance gets really laggy, especially if you've got an otherwise populated Chrome session running.
Offline, there's the FreeMind[2] tool, Java based. My main problem with it is that editing is incredibly fussy.
What I prefer mostly is GraphViz[3], which is not just a mind-mapping tool, but can be used as such. The advantage here is that what you're editing is a simple text file, with graphic output created by running one of the graphviz commands over it (dot, neato, twopi, circo, fdp, sfdp). For a traditional mind map, the 'circo' command is probably what you're looking for. While not as interactive as other methods, if you're working on a complex space, the lack of constant overhead for graphical presentation is a plus, as is the simplicity of the base language. What I'm still looking to do is to write a simple outline -> GraphViz converter which sets up edges based on indent levels and parent, which would further simplify the input problem.
________________________________
Notes:
Edit: The simplest tree editor is probably the file system on your computer. However, the problem is for proof's I suspect your going to want to have some sort of graph structure.
PS: If your willing to do some light coding XQuery/XSL-FO let's you transform the raw XML into various more user friendly formats like PDF's. However, if your structure is really nested that quickly get's hard for people to understand.