GUI tool for comparing directory trees

[Adam Bridge]

I maintain lots of image files on different drives and would like a
good tool to compare them to make sure I haven't NOT duplicated a
file. So a good GUI tool for comparing directory trees would be very
nice to have.

I looked at SmartSynchronize but they have licensing up the ying-yang
and it feels like a port from Windoze.

Anything better to use that you can recommend for making sure the
contents of tree A matches tree B?

Thank you!

Adam