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

I recommended the Minimoog VA VST to some people earlier:

http://home.no/gunnare/

A lot of fun if you have a Windows installation knocking about. Free VST host, if you need one:

http://www.hermannseib.com/english/vsthost.htm

Not quite the same thing, but it has the same level of accessibility in the interface - a testament to the engineers who put the Minimoog together.

I liked this brief history of the Minimoog:

http://www.youtube.com/watch?v=sLx_x5Fuzp4

edit

Similar (cross-platform) fun is to be had with the Bristol project's Mini:

http://bristol.sourceforge.net/mini.html

If you have an iPad I highly Moog's Animoog iPad app. Not a direct minimoog emulator, but it is excellent

fl studio from image-line is a pretty great stand alone synth sequencer that does the sort of soft synthesis shown here with aplomb