nessus-xmlrpc is a Ruby library for the Nessus XML-RPC interface. You can use it to start, stop, pause, and resume scans. It lets you watch progress and status of scans, download reports, etc. It comes with an example command line program that shows how easy it is to interact with the Nessus scanner.
PyroScope is a collection of tools for the BitTorrent protocol and especially the rTorrent client. "pyrocore" is the component providing command line tools for automation of common tasks, like metafile creation, and controlling rTorrent via its XML-RPC interface. It also offers a documented API, with which these tools are implemented.