Not all pretty and gooey, sorry, GUI
but how about using "ping" in a DOS window ?
Ping a host at the other end of the link under observation. Use the -l bufsiz option to set a max length buffer, and the -f "do not fragment" option to force it to maintain the packet size. Reduce the bufsiz until you don't get an error -- the bufsiz is then equal to MTU
Converting this to a small batch file for convenience is left as an exercise for the reader
