Recommended plugins

Filters and processing

• LSP plugins, an extensive set of plugins for effects and channel processing
• dpl.lv2, a look-ahead digital peak limiter
• fil4.lv2, a 4 band parametric equalizer with additional low & high shelfing and high & low-pass filters

Visualization

• x42 Meter Collection, a big help in analyzing audio
• sisco.lv2, an oscilloscope
• spectra.lv2, a spectrum analyzer

Drums and percussion

• AVL Drumkits, a drum sampler with two built-in kits
• DrumGizmo, a drum sampler with separately downloadable kits
• Geonkick, a drums/percussion synthesizer

Synthesizers

• Cardinal, a modular synthesizer with Eurorack aesthetics
• OB-Xd, a soft synth resembling vintage Oberheim OB-X synth
• Dexed, a Yamaha DX7 simulator
• Odin2, a 24-voice polyphonic synth
• Surge XT, a hybrid synthesizer
• Vital, a spectral warping wavetable synth

This website keeps a track of all software synthesizers available for Linux.