SpiralSynth Modular (or SSM) is an object orientated modular softsynth / sequencer / sampler. Audio or control data can be freely passed between the plugins, and is all treated the same. Data can also be fed back on itself for chaotic effects. LADSPA plugins are supported, and you can use them in the same way as any of the native plugins. Not having MIDI won't restrict what you can do. It currently uses the standard OSS/Free sound input/output (/dev/dsp), and should work on ALSA too. You can also render WAV files directly to disk.
Smurf is a GTK-based SoundFont editor. SoundFont files are a collection of audio samples and other data that describe instruments for the purpose of composing music. SoundFont files do not describe the music itself, but rather the sounds of the instruments. These instruments can be composed of any digitally recordable or generated sound. This format provides a portable and flexible sound synthesis environment that can be supported in hardware or software. Note that Swami is the successor to Smurf and is an entire object-oriented rewrite of it.
Spiral Synth is a physically modelled polyphonic analogue synthesizer. It is capable of creating the kind of sounds made by hardware analogue synths, the noises used in electronic music. You can also use it to make stranger sounds too. MIDI is supported, and it uses the standard OSS/Free sound output (/dev/dsp).