There is now a 'Synthedit like' app for OS X/AudioUnits: http://sonicbirth.com/
Have yet to try it though. Looks great but is very new.

For AudioUnits on OS X, Rax http://www.grantedsw.com/rax/ is a good alternative to the 'the Ugly VSTi Interface' mentioned.

For a modular 'audiomulch like' environment on OS X(windows too) there is Bidule: http://www.plogue.com/