MidiKeys is a little application that displays a piano keyboard onscreen. You can click the keyboard or use the computer keyboard to play MIDI notes. The keyboard window can be made to float on top of other applications and supports transparency. Also, MidiKeys will optionally set up hotkeys for the computer keyboard so you can play notes in other applications (such as a sequencer) without having to switch back to MidiKeys. It works great on a PowerBook!
SynthTest is a somewhat simple application for OS X that allows you to pick a MIDI source, a MusicDevice AudioUnit, and an Effect or MusicEffect AudioUnit to play with. You can open an edit window for both the synth and effect. If they do not have a custom edit GUI, the generic editor view will be used. SynthTest has a reference implementation of preset support for Audio Units. It is primarily intended to be used by Audio Unit developers for testing purposes (the source is available), although it can be great fun to play with all by itself. It is complete enough to be used for live performances.
Hydrogen is a simple real-time Drum machine/sequencer. It features a graphical user interface based on Qt, a sample-based audio engine, Jack/ALSA/OSS/PortAudio and Export to disk audio drivers, ALSA MIDI input, ability to import/export XML-based song and drumkit files, 192 ticks per pattern, 16 voices with volume, mute, solo, ADSR, and pan capabilities, and import of samples in wave, au, and aiff format.
Open Sound World (OSW) is a scalable, extensible programming environment that allows musicians, sound designers, and researchers to process sound in response to expressive real-time control. It combines a familiar visual patching paradigm with solid programming language features such as a strong type system and hierarchical name spaces. OSW also includes an intuitive model for specifying new components using a graphical interface and high-level C++ expressions, making it easy to develop and share new music and signal processing algorithms.
GENPO (GENeral Purpose Organ) is a program which replicates the essential features of any pipe, theatre, or reed organ. It reads a description of an organ (a .org XML file) and presents a suitable user interface for operating that organ. There are a number of high quality organ "SoundFonts" freely available and the .org file maps the stop sounds to organ manuals and stops. Other facilities such and couplers and presets are available. You may connect one or more MIDI keyboards to GENPO to provide the organ manuals, and you can even assign one to be the 'pedalboard' if so desired.
iabc is a graphical application for editing music. It uses the ABC music notation language to produce high-quality graphical notation or MIDI output. The music notation and the ABC notation can be updated in real-time, making the construction of tunes using the ABC language hassle-free (or at least hassle-resistant).
The Audio* Music Synthesis Project is music creation software. Currently it features Mojo Synth which is a polyphonic analog modelling synth that has a 64 step sequencer and can also be played from a MIDI keyboard. It attempts to emulate classic analog sounds from Roland M-100, Korg MS-20, and Mini Moog synthesizers.