An "equation" accessory might be useful - two uses spring to mind:
If the nodes were all the items required for some project, a cost could be attached to each and the sum of all the values in the child nodes shown in the parent.
Special escapes could be used in the node text:
"parent node $[SUM]"
--"child node1 text $[32.7]"
--"child node2 text $[12.7]"
when the parent node is not being edited it should display:
"parent node 45.4"
2) Decision making:
assign a value, +ve or -ve to various factors that might influence some decision, with the root showing tha balance.
e.g should I move house?
"SHOULD I MOVE? $[SUM]"
--"mail redirection hassle $[-0.3]"
--"more space $[+4]"
--"current annoying neighbours [+10]"
I'm sure I saw some discussion of how to somehow tie freemind in to a spreadsheet somewhere but I can't find any reference to it.