Your private keys are stored by a third party as well but they are encrypted with your password
Ease of use: Average
Supported coins: BTC OMNI
Private keys are never sent to the server except in an encrypted form.
You can deploy the Omniwallet on your own server and host your own instance, or use one of the service providers that will host it for you - your money, your choice.
Omniwallet comes with a pre-built support for Bitcoin, Omni and Test Omni.
An easy to use web interface for using mastercoin.