I've been working on a modular synth for a while, and it's getting to the point where it's very usable.
It's a modular 32-bit VSTi for Mac and Windows. Here's the link to the website:
http://straightarrow.dk/quiver/
There are still some stability issues, and it uses too much memory, but apart from that I feel pretty good about it!
If anybody has any thoughts or feedback, I'd be happy to hear it.

Ralph