Kraken is another that I released, but it's a lot more limited than Medusa2. I have a couple of unfinished ones on my HD, which I would finish and release if I wasn't so lazy. My main motivation for creating synths is so I can try new things with my music, so often times I'll just make something buggy for one track and then forget about it.
The VSTis I find myself using most aside from my own are Chip32, Triforce, and occasionally VOPM.
The main thing about Medusa's interface (aside from iLKke's gorgeous graphics) is that it packs the functionality I need the most into very few control elements. I was sick of having a monster like Syntendo open which didn't even have the stuff I needed, so that's why I want to keep it slim. For the next version, I'll try the custom arp, that vibrato delay and maybe support for pitch and mod wheels should anyone want to use it with a MIDI keyboard.
Oh, and I really like suggestions for improvement- as long as I agree they're a good idea, I'm happy to try my hand at it.