Bitsonic TB-303

Bitsonic TB-303 is a poly/monophonic bass line synthesiser with a single digitally modelled analogue oscillator, filter and decay envelope.

 http://www.bitsonic.eu/tb303.html

Audio Desk MB3X

Monophonic bass synthesizer

MBX3 by Audio Desk – inspired by the legendary analog monophonic MAM MB33 mkII bass synthesizer. With the very similar layout as the original, the MBX3 has one oscillator with sawtooth or pulse waveform and a sub oscillator with sine waveform, followed by an analog-modelled 24db lowpass filter with modified resonance reaction and passive-controlled overdrive circuit inside the filter. Like the original the MBX3 has a distortion fx unit on the signal output for more aggressive sound shaping. All the features united with 2x oversampling and low cpu demand. 

http://www.audio-desk.com/free-vst-synthesizer-audio-desk-mb3x/

Venom VB-303

Venom VB-303, a brilliant free Roland TB-303 emulation
http://www.kvraudio.com/forum/viewtopic.php?t=380007