This is a great tool, thanks! It really helped me to figure out the InSim traffic, before taking the plunge into InSim programming.
One minor bug: after I moved the location of the LFS installation, InSimSniffer stopped working. It exited with a null pointer exception at startup. (Can send you the details if you want.) Apparently, InSimSniffer assumes that when it finds an LFS registry key, it will also find the LFS install at that location.
Improvement suggestion: Put the filter settings in a different sidebar, so the user doesn't have to switch tabs between filter settings and message list. (Even nicer: apply filter changes immediately, without needing to click "Apply".)