I know Sinthedit for a long time and used it.
The great thing is that you can "design" your own synth or FX processor and create it as a VST plugin, then load it into your favorite host.
we had a thread at The Brewery dedicated to VST plugins (and Synthedit) so maybe you can find something interesting there:
http://www.groupdiy.com/index.php?topic=2690