If you want something that isn't a toy and emulates all the best features of analogue synths, try AlsaModularSynth -- http://alsamodular.sourceforge.net/ http:/

If you want something that isn't a toy and emulates all the best features of analogue synths, try AlsaModularSynth -- http://alsamodular.sourceforge.net/ http://alsamodular.sourceforge.net/ -- also available as amsynth in many Linux repositories. It's got demos that emulate Moogs in all their glory. Steep learning curve as you'd expect from a full modular synth, but lots of samples and demos, and works with MIDI so you can play it with a real keyboard.