Add64 (Standalone)
Add64 is the result of experiments around additive synthesis and is intended for research purposes only.
The spectral drawing scheme for the harmonics and envelopes has been inspired by the virtual pipe organ Aeolus by Fons Adriaensen.
Project Page: https://sourceforge.net/projects/add64/

amsynth (Standalone + DSSI + LV2)
amsynth is a software synth that provides a classic subtractive synthesizer topology.
Project Page: http://code.google.com/p/amsynth/

Calf studio gear (Standalone + LADSPA + DSSI + LV2) [*]
The Calf plugins include all todays frequently used studio effects along with some feature-rich sound processors.
Homepage: http://calf.sourceforge.net/
[*] This is a custom version which adds back LADSPA, DSSI and LV2 external-UI support removed by upstream, see this link.

CAPS (LADSPA)
The C* Audio Plugin Suite (CAPS) comprises a range of classic sound effects and a few more basic audio DSP units, signal and noise generators.
The suite's aim is to provide impeccable sound quality, computational efficiency and no perceptible latency.
Homepage and Documentation: http://quitte.de/dsp/caps.html

drumkv1 (Standalone + LV2)
drumkv1 is an old-school all-digital drum-kit sampler synthesizer with stereo fx.
Homepage: http://drumkv1.sourceforge.net/

Fomp (LV2)
Fomp is an LV2 port of the MCP, VCO, FIL, and WAH plugins by Fons Adriaensen.
There are 13 plugins in total: 1 auto-wah, 1 EQ, 3 chorus, 5 filters, and 3 oscillators.
Homepage: http://drobilla.net/software/fomp/

LinuxSampler (DSSI)
Free, streaming capable open source pure software audio sampler with professional grade features,
comparable to both hardware and commercial Windows/Mac software samplers.
Homepage and Documentation: http://www.linuxsampler.org/

LinuxSampler (LV2, 2 channels)
Free, streaming capable open source pure software audio sampler with professional grade features,
comparable to both hardware and commercial Windows/Mac software samplers.
Homepage and Documentation: http://www.linuxsampler.org/

LinuxSampler (LV2, 32 channels)
Free, streaming capable open source pure software audio sampler with professional grade features,
comparable to both hardware and commercial Windows/Mac software samplers.
Homepage and Documentation: http://www.linuxsampler.org/

LinuxSampler (VST)
Free, streaming capable open source pure software audio sampler with professional grade features,
comparable to both hardware and commercial Windows/Mac software samplers.
Homepage and Documentation: http://www.linuxsampler.org/

MDA (LV2)
MDA-LV2 is an LV2 port of the MDA plugins by Paul Kellett.
It contains 36 high-quality plugins for a variety of tasks.
Homepage: http://drobilla.net/software/mda-lv2/

REV-Plugins (LADSPA)
Reverb LADSPA plugins.
Includes a stereo reverb plugin based on the well-known greverb, and LADSPA versions of zita-rev1 in resp. stereo and first order Ambisonic mode.
Homepage: http://kokkinizita.linuxaudio.org/linuxaudio/index.html

samplv1 (Standalone + LV2)
samplv1 is an old-school all-digital polyphonic sampler synthesizer with stereo fx.
Homepage: http://samplv1.sourceforge.net/

synthv1 (Standalone + LV2)
synthv1 is an old-school all-digital 4-oscillator subtractive polyphonic synthesizer with stereo fx.
Homepage: http://synthv1.sourceforge.net/