Marvin has an open source command-line interface (available here) which you can use to easily interface with the public API, as well as the local desktop API server.
This lets you open a terminal/command prompt on your computer and type in:
marvin add "Take out trash +tomorrow at 8am"
This is useful for users who frequently find themselves in a terminal, or for users who want to script Marvin (without writing their own bindings for Marvin's API).