*** This plugin has now been moved to it's own thread - download from the link below ***
It is a rather specific plugin but perhaps it will be useful for someone.
The plugin enables sending of books from Calibre using SSH (or RSH) protocol. It requires rsync or similar utility to be installed in the system. It can be used to send books from Calibre wirelessly to your ereader or another computer with SSH server running (on Kindle, this can be achieved by usbNetwork hack; most other readers run Linux too, so it should be possible to install SSH server). The plugin enables customization of the rsync (or equivalent) utility path, remote host details, formats of books to be sent + a Kindle specific option of refreshing the book list after sending. The advantage of rsync is that it only sends the differences between the Calibre book and the book existing on the remote host, so if the book already exists or if only metadata have changed, there will be (almost) no transfer.
The implementation is simple, so it has a few shortcomings currently: Calibre cannot be used while books are being sent by SSH; the button is not integrated into the "Send to device" group; applicability on Windows is questionable (no command line rsync utility for Windows as far as I know?), thus I disabled the plugin for Windows by default; only one remote host at a time can be specified at the moment; only SSH login with preshared keys is currently supported. Some of the shortcomings could possibly be sorted out on request - I did not want to spend too much time implementing without knowing that somebody would actually use it.
First follow the general instructions from kiwidude mentioned at the beginning of this thread: download the zip file, install the plugin, add the 'Send by RSYNC' button by customizing the toolbar.
Finally, customize the plugin in Preferences->Plugins->User Interface Actions Plugin->Send by RSYNC. A Calibre restart seems to be necessary for any Calibre plugin customization to take effect.
EDIT: kiwidude, could you perhaps add link to this plugin in your first post? thanks