Omni Wallet offers a Web-based interface for checking balances, as well as a Web wallet and decentralized trading using Master Protocol.
Your Bitcoin private keys are stored by you and not on the website.
The MasterCore Wallet Beta has been released supporting trading MSC/BTC on the Decentralized Exchange, and support for Smart Property balances and SP sending.
MasterCore offers a full fledged reference client based interface similar to Bitcoin Qt.
If using the wallet for the first time please read the instructions on Github.