SerialSpy acts as a serial pass-through device. It listens for incoming data on two serial ports and forwards it so the devices act as if they are directly connected. It also logs the data as it moves through the ports.
nauta is yet another serial browser, primarily for GPS receivers. It consists of several distinct
applications (like ZboxZ), so it is expected to be rather slow (but usable), hard to install, and flexible. It also supports DGPS.