I just launched mcoin.io, a self-service Mastercoin order book. It will hopefully replace or at least complement the manually managed Google order book.
As soon as the distributed Mastercoin exchange is fully working, mcoin.io most likely will become obsolete. But I hope it helps people to buy and sell Mastercoins in the meantime.

Some features:
- You can add and remove your own sell / buy orders (no need to wait for manual updates)
- Balances of addresses get verified, fake orders can't be created
- You can create prefunded orders and edit prices yourself
- Prefunded orders can be bought directly from the site, no need for direct contact or escrow (as long as you trust me). Sending of Mastercoins will be done manually though, no private keys or funds are stored on the server.
- No need to create an account at the site and no passwords to be lost or stolen. Everything is based on signing messages with your Mastercoin / Bitcoin address.

I don't know how to sign a message with my address
I created a little tutorial to help you get started with this: http://mcoin.io/#/help

What about the manual Google order book?
From now on, I'm asking everyone to submit their orders to the new self-service order book at mcoin.io. The manual Google order book will still be accessible for a while but I'm strongly encouraging everyone to test the new order book.

Bugs and suggestions
The new site is still in beta and I guess there's lots of room for improvements. Please let me know about bugs, typos and suggestions.