The only decent tool I'm aware of is the editor for Neverwinter Nights (in their Aurora editor). The only other talk I've heard about other tree editors
is here, but unfortunately the tool referenced there is no longer available as far as I know. If you go to the NWNvault you can find a list of conversation related programs; go to
http://nwvault.ign.com/, do a site search for 'conversation' then look down the results for the link labeled 'Other'. There looks like a couple of possibilities there but I'm not sure what will be helpful, sorry.
Since you mention it this kind of tool is something I've thought about having a go at, but I haven't done any work on it so far. What kind of features would you want?