A bass synthesizer introduced by Roland in 1981 and intended to complement the TR-606 drum machine. Both the 303 and 606 had built-in sequencers; they were intended to generate accompaniment to, for instance, allow a guitarist to practice alone. Like the 606, the 303's build quality was dodgy and it sold poorly at the time.
However, around 1990, Chicago house artists discovered that the TB-303 could be made to generate some very unique bass sounds if the voltage controlled filter circuitry were to be overdriven a bit. The 303 employs a unique and oddball configuration of a diode ladder filter, which can produce a very characteristic sound. The 303's sound became the cornerstone of its own electronica genre -- acid house. Artists and DJs learned how to sweep the filter's cutoff frequency and resonance to produce the "moving", highly resonant sound associated with acid house.
Like the TR series of drum machines, the TB-303 has given rise to numerous clones, such as the Novation Bass Station. The perfection of clones is much debated, especially the filter circuit. An analysis of the filter performed by Tim Stinchcombe in 2009 shows a surprisingly complex response curve, including an unusual peak in the resonance circuit at 10 Hz. The filter is frequently described as being a three-pole, 18 dB/octave circuit, but Tim's analysis shows that it is in fact a four-pole filter.