I mostly use vintage pedals, but the one modern pedal that I really like is the Electro-Harmonix Tube Zipper (
Electro-Harmonix Tube Zipper: Harmony Central User Reviews - page takes forever to load)
It can do Mutron-type envelope stuff, but it's got way more to it. Its got 2 tubes that the signal gets filtered through. It's really complex and I haven't even scratched the surface on what it does, you do need to read the manual to get started. The sound is really nice. I paid around $140 on ebay. It's the kind of thing that you can get a lot of different sounds out of and they all sound good.