Stand-alone filters are relatively rare. A short list of static & variable filters plus a few outliers

Static EQ-style filters

• Pultec HLF-3C passive low-pass & high-pass filter (obsolete)
• Harrison Ford filter DIY clone of the filter from a Harrison console
• TK Audio TF1 500-series filter (discontinued)
• Wavetek Filter Set

Dynamic Synth-style filters

• Sherman Filterbank – the original
• Sherman Filterbank 2 – new model of the classic filter / distortion unit
• Akai MFC-42 three-channel filter / effects unit
• Mutronics Mutator two-channel filter with lfo & triggered sweeps
• Ebbe und Flut Schippmann’s über filter box
• Elektron Analog Heat II
• Electrix Filter Factory stereo filter unit
• Waldorf 2-Pole Analogue Filter – stand-alone synth-style mono filter
• FAT PCP330 Procoder Filter Bank – multi-frequency filterbank
• MAM VF11 Vocoder Filterbank
• FAT THC-01 Resinator Filter – more MAM / FAT filter madness
• MAM Analog Filterbank 8
• Bugbrand Bugcrusher
• Jomox M-Resonator
• Peavey Spectrum Filter
• Jomox T-Resonator
• Wavetech 442 Dual Filter

Dynamic & Programmable Equalisers

• SSL SL683E programmable equaliser
• BSS DPR-902 Dynamic EQ

Modwiggler filter thread

https://modwiggler.com/forum/viewtopic.php?t=50143&start=10

Reddit Synth outboard filter page

Could we list all the Filterboxes we know?
byu/Seri05 insynthesizers

Peavey Spectrum Filter #1

MAM Warp Nine Analogue Midi Filter #2

Wavetek Model 442 Filter Set

Electrix Mo-FX

Jomox T-Resonator II

Analog Heat Mk II

Sherman Filterbank 2 Compact

Mutronics Mutator

Akai MFC42

MAM PCP330 Procoder

MAM VF11 Vocoder

FAT THC-00 Resinator Filter

Waldorf 2-Pole Filter

Bugbrand Bugcrusher 08

MAM Filterbank 8

Filter Factory Mk2