As a solo artist and then studio collaborator, I was always interested in getting as many varieties of sound from my bass as possible... the further out the better, and the best being the sounds that no one would assume was a bass at all. I began to look very studiously into how to sound like a synthesizer, which led me to believe that what makes analog synths sound the way they do, by and large, is the feeding of a simple tone into a filter. There's a little more to it than that, but the filter is the key.
Then the hunt began-- I bought my first wah simply because it was a filter, and why would I balk at having a foot-controllable band-pass filter in my arsenal? I found out about multi-mode filters, and the envelope followers... eventually I picked up what I needed most, a stompbox, foot controllable, low-pass filter... the Moog. Everything else is gravy.
Moog MF-101 Lowpass Filter
Electro-Harmonix Bass Balls (modded)
Dunlop Crybaby 535Q
Digitech XP-1000 (jetpackmods)
Electrix Filter Factory
DOD FX-17 Wah/Volume
Danelectro Fish & Chips 7 Band EQ