
KMidimon is a MIDI monitor for Linux using ALSA sequencer and KDE user interface.
KMidimon monitors events coming from MIDI external ports or applications via the ALSA sequencer, and from SMF (Standard MIDI files). It is especially useful if you want to debug MIDI software or your MIDI setup. It features a nice graphical user interface, customizable event filters and sequencer parameters, support for all MIDI messages and some ALSA messages, and saving the recorded event list to a text file or SMF.
GMidiMon is a similar program with GTK user interface for OSS MIDI devices.
Report problems, bugs and feature requests to the author or the tracking system at SourceForge project site
Copyright (C) 2005-2009, Pedro Lopez-Cabanillas <plcl AT users.sourceforge.net>
There is an excellent tutorial in French here: Linux MAO. KMidimon: moniteur de messages MIDI
And this is the automatic translation by Google: Linux MAO. KMidimon (English version)
Thanks a lot to Mysth-r!
You can control the ALSA sequencer connections to programs and devices from inside KMidimon. To do so, use the options under the menu "Connections" in the main menu. There are options to connect and disconnect every available port to KMidimon, and also a dialog box where you can choose the ports to be monitored one by one.
You can also use a MIDI connection tool like QJackCtl to connect the application or MIDI port to monitor with KMidimon:

To start and stop monitoring, use the buttons "record" and "stop" in the toolbar, or the corresponding menu items under the "Control" menu. After enabling the monitorisation, KMidimon will show incoming MIDI events if your setup is correct.
You can get the latest released sources from SourceForge.
There are ready to install packages for:
In order to successfully use KMidimon, you need KDE 4.X. and ALSA 1.0 drivers and library.
ALSA library, drivers and utilities can be found at ALSA home page.