Rubberduck is real-time synthesizer designed to help you produce electronic dance music. Loosely based around the TB-303 synth, the application consists of a wave oscillator with a filter for applying sweeping and resonance. Everything is packaged in an intuitive manner, meaning that it's pretty easy to start creating banging house, trance and techno music (as long as you have an ear for dance music, of course).
It incorporates many interesting characteristics such as: 24 dB filter, a totally configurable interface, support for skins, dynamic real-time resonance filters, four classic waveforms, dual generator, delay, distortion and many other effects. If you've never produced dance tracks before then you may find Rubberduck a little daunting but for production dab-hands, the program is pretty good.