U-he Zebralette is available for free now. Zebralette is the little brother of the great synthesizer Zebra 2, and has one oscillator, two LFOs and GeoMorph, a feature that lets the waveform of the oscillator gradually morph into another waveform. Furthermore the synthesizer is equipped with a chorus, an equalizer and a delay.
If you don’t have Zebra 2, then it is definitely worth downloading Zebralette. This free VST-plug-in has the same great sound quality as Zebra 2. Despite that it only features one oscillator, it could easily become one of the most popular free synthesizers.