I use several plugins on different sites which I developed for private use only. This is for development reasons and/or because they are very specific for these sites. However I missed a possibility to update them in a convenient and quick way. So I did some research and found a great solution in using the Infinite WordPress Panel and the custom plugin and theme update libraries by White Shadow.
Infinite WP Panel lets you control all your blogs from one place. You need to install the IWP Client on the sites you want to control remotely first. After activation of the plugin it will show you the credentials that you need to add the site to your central wordpress control panel. This is definitely a nice way to work with larger numbers of completely independent wordpress sites when you do not want to mess around with the multi site option. Especially maintenance of multiple client websites is much easier this way because usually they are all hosted on their respective webspace.
White Shadow’s libraries for automatic updates for private and commercial themes and updates for private and commercial plugins work perfectly together with IWP thus giving me the opportunity to develop a private plugin, put it into my own repository and do a one click update for all the sites that use it.