Helge Schroeder of FSNavigator fame released the successor to FSNav for use in FSX, FSTramp.
If you have missed FSNav in FSX go to the site and check it out. You can try it for free for 20 sessions.
http://www.fstramp.com/
[EDIT]
I just had a quick look and you can not use it as a flight planning tool. It takes the plan loaded in FSX. I am confused about what possible use this program has that makes it worth 30 euro's?
[\EDIT]