Is there any service/api for deriving HD wallet addresses from a master public key?
I ended up writing my own command-line tool to perform wallet discovery and print out a report.

It supports single xpub derivation as well as multisig m-of-n wallets, specifically CoPay wallets.

It determines if each address has been used or not by querying blockchain.info or also toshi or insight servers (the latter two can be run locally.)

Anyone interested can check it out at:
https://github.com/dan-da/hd-wallet-addrs

I plan to add a web frontend soon.  Update: Web frontend is here.

Also, blockonomics is pretty good and very fast.  They don't seem to support CoPay/multisig yet though.