I really don't care much for decentralized. I think Open Source is more important, and there is really nothing.
I've looked at Peatio and didn't like it for a few reasons:
1) It's in Node.Js, PHP is preferred.
2) Too many internal dependencies (See #1)
3) It's ugly
Then this Cryptsy clone (
https://github.com/crypto-maniac/Cryptsy-Clone). I briefly looked, but still not something I will use long term. However I think it's a acceptable basis to work off with some effort.
1) Low quality/Sloppy code.
2) No trade engine
Make something that doesn't requiremen to download thousands of external 3rd party libraries, looks nice, and is written in PHP.
I'm sure it will catch on.