TerminatorX is a realtime audio synthesizer that allows you to "scratch" on digitally sampled audio data (*.wav, *.au, *.mp3, etc.) the way hiphop-DJs scratch on vinyl records. It features multiple turntables, realtime effects (built-in as well as LADSPA plugin effects), a sequencer, and an easy-to-use GTK+ GUI.
Soundtank is a program that hosts musical instruments for you to play; each instrument can be controlled via MIDI. You can create custom instruments and save them to files, and can also load instruments that other people create. It relies on LADSA plugins, as it has no built-in sound synthesis algorithms, but hosts them in "realtime objects" which embody the structure of the audio signal flow. RTObjects can be controlled in a completely customizable fashion using MIDI events sent through the ALSA sequencer interface. Soundtank outputs audio through ALSA or JACK.
The Advanced Linux Sound Architecture is composed of several parts. The first is a fully modularized sound driver which supports module autoloading, devfs, isapnp autoconfiguration, and gives complete access to analog audio, digital audio, control, mixer, synthesizer, DSP, MIDI, and timer components of audio hardware. It also includes a fully-featured kernel-level sequencer, a full compatibility layer for OSS/Free applications, an object-oriented C library which covers and enhances the ALSA kernel driver functionality for applications (client/server, plugins, PCM sharing/multiplexing, PCM metering, etc.), an interactive configuration program for the driver, and some simple utilities for basic management.
Twibright Syntring is a purely algorithmic string synthesizer with ASCII tab input and wav output. It doesn't use any samples, yet sounds quite realistic. Timbre, volume, and timing are adjustable in the tab file. It can be used for learning or practicing guitar or bass playing, for editing and synthesizing one's own tunes, or producing MIDI sound fonts. It is very lightweight and portable, and doesn't require any libraries.
Algoscore is a graphical environment for algorithmic composition, where music is constructed directly in an interactive graphical score. The result is output to MIDI and CSound. Graphical objects are placed in a timeline and connected together. Some objects are fully dependent on user data, while some are generative and reacts on input from other objects. It is highly customizable and extensible with the Nasal scripting language. It has a non-realtime perspective where the composer can relate freely to time and construct the composition outside of time. This makes AlgoScore more like a traditional sequencer in this regard, but a graphical algorithmic sequencer with powerful and flexible scripting capabilities.
Qsynth is a GUI front-end application for Fluidsynth. Fluidsynth is a command-line software synthesizer based on the Soundfont specification. Qsynth was built with the Qt4 toolkit using Qt Designer. Eventually it may evolve into a softsynth management application that allows the user to control and manage a variety of command line softsynths, but it currently just wraps the excellent Fluidsynth program.
Beast is a powerful music composition and modular synthesis application. It offers multiple input methods like multitrack, piano roll, and pattern editing and supports skins. On the technical side, it has a wide range of abilities like sequencing, unlimited undo/redo history, real-time synthesis with multiprocessor support, full duplex 32-bit audio rendering, precise timing down to sample granularity, on demand loading of partial wave files, on the fly decoding of various sample formats, aliasing free oscillators, and full Scheme scripting support.