KMidimon monitors events coming from a MIDI external port or application via the ALSA sequencer, or 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, MIDI playback, and saving the recorded event list to a text file or SMF.
Desfonema Sequencer is a tracker-inspired MIDI sequencer. It uses a pattern oriented design, with n tracks by pattern. It offers a piano roll editor for tracks, and a tabbed track interface for patterns. It features convenient keyboard shortcuts, a virtual keyboard, step and live editing, MIDI input (ALSA), and a text-based file format that is easy to edit manually or merge and better suited for version systems like Subversion.
hexter is a software synthesizer that models the sound generation of a Yamaha DX7 synthesizer. It can easily load most DX7 patch bank files, edit those patches via a built-in editor or MIDI sys-ex messages, and recreate the sound of the DX7 with greater accuracy than any previous open source emulation (that the author is aware of). It operates as a plugin for the DSSI Soft Synth Interface.